Terumi's "birth" was when the Susano'o Unit gained a soul. Over time, Terumi grew sick of being shackled to the endless cycle of destruction and creation that he shared with the Amaterasu Unit. He allowed the Susano'o Unit to be found by humanity and separated his soul from the unit, inhabiting the body of a young man with green hair. He contacted Relius Clover and Shuichiro Ayatsuki with the promise of creating a perfect Prime Field Device - a Kusanagi - it went haywire and a Black Beast was created, starting the Dark War. Or so Terumi believed. In actuality, the Black Beast was a time-displaced Ragna the Bloodedge and Nu-13. At some point, he became acquainted with Clavis Alucard and Valkenhayn R. Hellsing.
Phase Shift 1, Phase Shift 2, Phase Shift 3, Phase Shift 4
Terumi lost his body as the Black Beast arose, and demanded to Relius to create him a body. Before the scientist was swallowed by the Beast, he told Terumi that a vessel was inside the Magister's City of Ishana waiting for him. Terumi managed to penetrate its barrier, creating a body for himself through self-Observation.
In time, Terumi found the vessel, Kazuma Kval, and waited in his dorm for the teenager to wake from his sleep. When Kazuma awoke, Terumi introduced himself and asked if he wanted the Azure. He mysteriously left without much explanation on what the Azure is.
Watching as Kazuma interacted with Trinity Glassfille in the Great Library, Terumi mocked Kazuma afterwards and began to walk back to the boy's dorms with him. He openly mocked Trinity and told Kazuma to not get corrupted into becoming a mage. Terumi told Kazuma that he had to get the Azure by himself and they entered a park. There, Kazuma was assaulted by three half-beastkin, but Terumi nonchalantly sat on a bench and crossed his legs, applauding whenever the student dodged an attack. Instead of helping him, Terumi teased Kazuma until the student promised that he would find the Azure.
Rising from his sitting position and grabbing a nearby throwing knife that one of the half-beastkin had lodged in a tree, Terumi smiled. He ran towards the assailants and countered every attack they threw at him, repelling the group. As they retreated, Terumi threw the daggers on the floor, uninterested in them. He separated with Kazuma outside of the boy's dorms and looked over his shoulder, seeing Tomonori in the distance. Later that evening, Terumi entered Kazuma's dorm and warned him that Trinity was monitoring him.
The following day, Terumi watched as Kazuma was in a cafe with Nine, Trinity, and Celica A. Mercury. Catching the boy's attention, Terumi jerked his chin and walked onwards - Kazuma ran out of the cafe after him, finding the man near the edge of Ishana. He pointed out a hole in the Ishanan barrier, telling Kazuma that with this he could distract the Ten Sages. As soon as Trinity and Nine appeared, Terumi disappeared.
Kazuma had made it to the center of the Ishanan Cathedral, the Lost Gate. He taunted Kazuma deeper and deeper and asked him to put his hands on the cauldron beneath. Kazuma's memories came flooding back to him, and Terumi physically appeared - Terumi spoke, telling Kazuma more about his origins as a vessel, and the pair's shadows stretched to meet one another. As they began to fuse, they turned to see Tomonori staring at them.
Terumi watched as Kazuma battled against Tomonori and as the beastkin drew the Hihiirokane to fight them both. Together, the duo summoned a version of the Geminus Anguium: Ouroboros - Tomonori lost and was viciously killed by Kazuma while Terumi laughed and applauded. A brief while passed and their fusion had become complete, although both were exhausted from the fight. Turning around, they were found by Valkenhayn, who knocked them unconscious and dragged them to the Alucard Castle. They were locked behind several sorcery barriers and shackled with binding chains by Clavis. Months passed and Terumi and Kazuma had become mirror images of one another in their isolation.
After the death of Clavis, Nine, Jubei, and Valkenhayn entered the prison. Nine interrogated Terumi, who wildly laughed at her, but fell silent when she used Mind Eater on him, turning the God into her slave. In the months following, Terumi was taken to Ishana where he became a member of a group of warriors which included himself, Nine, Trinity, Valkenhayn, Jubei, and Hakumen. His knowledge on seithr, the Black Beast, and cauldrons was utilized to help the war efforts and develop Ars Magus. These group of warriors would be known to history as the Six Heroes. Because of Mind Eater, Terumi's actions were limited, and Nine could find his location on a whim, so he spent most of his time alone in one of the Guild's housing complexes.
Nine called a meeting between the warriors. Terumi gleefully taunted Nine, but was forced to be quiet upon her command. When Hakumen was discovered missing from the council, they ended it prematurely and left the meeting room. He followed the group and saw Nine with Celica, and began mocking them both while they embraced. Hakumen grabbed him by his collar, and Terumi turned his attention to the knight, mockingly asking if they were friends. Celica managed to stop the quarrel - Terumi avoided her touch when she attempted to grab his arm. Seven and Eight were not fond of the group that Nine had brought into the Guild, and so they disbanded with Terumi leaving under the guise that he was leaving to use the toilet. Kazuma then took control of the body and approached the Sages, interested in the plans they had for Gigant: Take-Mikazuchi.
Eventually, Terumi made his way to the cafe where Trinity, Jubei, Celica, and Nine were. He ordered a black tea and a hard boiled egg and mocked Nine for asking him where he had been. Under Mind Eater, Terumi could not lie to the witch, but since it was Kazuma in control and not Terumi during his encounter with both Sages, Nine was none the wiser about the interaction. He sat away from the others, but was made to apologize to Trinity after Jubei and Celica accidentally made her glasses dirty. When they left the cafe, Nine kicked Terumi as hard as she could in his stomach after he made a comment about her being deluded. As Trinity helped him to his feet, he whispered something unknown into her ear and left for the complex in the Mage's Guild where he spent most of his time.
Alongside the other Heroes and Celica, Terumi was part of a meeting that concerned ambushing the Black Beast when it arose from its one year slumber. He told them the location of every cauldron in the world, also pointing particular attention to the Beast's nature to hunt out the things with the most life-force. They agreed to ambush it in Eastern Europe with Celica as a bait for the Beast, and so preparations were made for the battle ahead.
Terumi was in charge of one battalion that was comprised of Ishana's mages. When the Beast finally arrived, he manically laughed, but only because he could think of no other response. Under his breath he noted that the Beast was much larger than the time when it killed his previous vessel. Terumi watched as Seven and Eight summoned the Take-Mikazuchi to successfully repel the Beast. As every combatant ran towards the center where Celica lay unconscious because of the weapon, Terumi stood alone - he laughed loudly to himself, thanking the Sages for bringing him closer and closer to his goal.
Terumi returned to Ishana, allowing Kazuma to take control so he could intimidate Seven and Eight free from Nine's ever-watching eyes. When Kazuma left, he allowed Terumi to take over. He waited in the Ishana Cathedral until Valkenhayn, Hakumen, and Jubei arrived. Trinity and Nine later joined them and the two mages began to pray to an idol of The Creator – Terumi mocked them, finding it absurd that they both prayed to a God. Nine ignored him and began to relay to them a version of the Prime Field War, which Terumi interrupted with a yawn. Ignoring him once more, she continued to detail the origins of Take-Mikazuchi, Hihiirokane, and the Sealed Weapon Izayoi. Before she could continue, however, she was arrested by the OLN, who were led by Seven and Eight. Terumi watched as she was taken to her trial. 
Moments later, a flood of OLN soldiers entered the building, announcing that the heroes were under arrest. Terumi didn't care, and watched as Hakumen effortlessly bested the battalion in under a minute. Even Terumi was impressed by this, letting out a whistle before snapping Trinity back into focus after she spaced out. Hakumen had already left the group to find Celica, so the group decided to return to Trinity's workshop where they could hide for the time being. When in the workshop, both Jubei and Valkenhayn expressed their disbelief that Terumi had willingly come with them, but he sarcastically responded that he was just doing his job. Before the group could decide what to do next, a large explosion rung throughout the city.
They ran towards the source of the explosion, finding Nine locked in battle against Seven and Eight. Terumi laughed, finding it both hysterical and fascinating that members of the Ten Sages would be battling one another. As OLN soldiers and various other members of the Sages began to make their way to their location, Terumi suggested killing every last one of them, only to be stopped by Trinity. Instead, the group retreated with Nine in tow until they came across two shadowy figures that Jubei knew. These figures guided the group away from the pursuers into a secluded forest where Nine was able to regain her magical energy. Renewed, she teleported them away from Ishana and into another location.
Terumi awoke in a location not too far from the Kaka Village. Nine began to shout at Hakumen to let Celica down and Terumi mocked her, comparing her to a barking dog. Nine immediately kicked him to the ground. As he got up, the two shadows introduced themselves as Suzukaka and Totokaka of the Kaka, Jubei's clones. The group was guided by the two girls to the hidden Kaka village. In the evening, Terumi continued to mock Jubei and Nine by commentating on their relationship and how sick it made him. Despite this, Nine continued her earlier explanation on Logic, the Age of Origin, the Black Beast, and Take-Mikazuchi.
In the evening, Terumi found Hakumen moongazing atop a boulder. He climbed up next to him and began to mock him. Terumi knew of Hakumen's origins and started pestering him, calling him Jin Kisaragi and asking which phase of the Looping World he was from. This brought up painful memories for Hakumen, who threatened to kill Terumi on the spot, only relenting when he remembered how critical he was for defeating the Black Beast.
The following day, the heroes, as well as Suzukaka and Totokaka, were climbing boulders near the village. Trinity slipped and Terumi, perhaps influenced in some way by Kazuma's personality, helped her up. They followed Celica and Hakumen towards an old train station. The tracks led into an old tunnel, but upon approaching it, Terumi began to feel sick. Nine ordered him to stay outside and Trinity stayed with him.
Later that same day, Terumi was with the other heroes as they discovered the OLN and Ten Sages were going to use tens of thousands of people in a village in Asia as a sacrifice to summon Take-Mikazuchi. The group decided to stop them before their plan could go ahead. The group, alongside Celica, Suzu, and a group of Kaka, where teleported to the exterior of the village. Terumi was ordered to sneak around to Seven and Eight and attack them, with Mind Eater compelling him. Terumi ran towards them, but he was not fast enough as he was too late to stop them both from summoning Take-Mikazuchi. When the Beast and Take-Mikazuchi fought and disappeared back into Ishana through a magic circle, Terumi was nowhere to be seen. 
Two days after the fight in the village, Terumi finally made his way back to Ishana. There, he saw that Take-Mikazuchi had crashed into the island, destroying its center and injuring many civilians. One of the many heads of the Black Beast had also been dragged into Ishana as well, making the monster aware of Ishana's location within the world, thus putting it in grave danger. When Terumi came back, the OLN and Mage's Guild were no longer hunting him and the other heroes down as he was considered a lower priority compared to both restoration efforts and the rest of the war. Nine summoned him to a meeting with the other heroes, where she revealed that she had taken half of Take-MIkazuchi's core and would be creating new weapons with it, the Nox Nyctores. These weapons would take her time to complete, however, and that was time that Ishana simply did not have. Terumi listened, but did not contribute, as the team created a plan to stall the Beast at multiple cauldrons before it would reach Ishana.
Terumi and the other heroes, bar Nine, helped lead the efforts to stall the Beast. Terumi's own magical ability was helpful in setting up various barriers. He found Trinity among the camp set up near the barrier and began his end plan for her manipulation. He told her that they were comparable: both slaves to Nine who could never leave her shadow. Terumi also made Trinity aware that she could undo the Mind Eater he was subjected to. After leaving her with that information and telling her that he knew everything that Nine was hiding in secret, he left, which left Trinity confused over her relationship with Nine.  He, among other mages, returned back to Ishana to assist Nine with the development of the Nox Nyctores. The plan was a success and humanity was able to buy enough time for Nine to complete the weapons, although many lives were lost to stall the Beast.
Terumi was gathered with the other heroes by Nine to a workshop within the Mage's Guild. She presented each of them a Nox. To Trinity went the Arma Reboare: Muchourin, to Jubei the Mucro Somnio: Musashi, to Hakumen the Interfectum Malus: Ookami, to Celica she returned the Deus Machina: Nirvana, and to Terumi she gave the Geminus Anguium: Ouroboros. Valkenhayn refused to accept a weapon, preferring to instead fight the Beast with his own body. Terumi whispered under his breath that this was the real Ouroboros, confusing Nine who did not understand that he had somehow wielded a version of it years prior against Tomonori. Nine then explained that the weapons would all be sealed at the end of the war as their power was too much for humanity and could end up destroying the balance of the world.
Ishana rallied together with the remainder of humanity to strike the Black Beast for one final time. Hakumen summoned the other heroes and Celica to Trinity's workshop. There he revealed that they were not going to be able to kill the Beast. Their mission was simply impossible unless they tried something else. To this end, Hakumen proposed that Celica go inside the Beast and meet with Ragna. Together, they could fight against the heart of the Beast, Nu-13, causing the Beast to lose some of its intelligence and succumb to its base desires of wanton massacre. It would be baited to follow after a large army, bringing it closer to the Kaka Village and the cave near it which was filled with crystals of Order. These crystals neutralize seithr, so the Beast will only get so close. Once it was close enough, the Ten Sages and strongest members of the Mage's Guild would work together to teleport the Beast above it. The Black Beast would be drastically weakened and could then be killed. In order to do so, however, both the body and the heart would have to be killed at the same time as they shared a life-link. Jubei offered up a solution to get Celica inside the Beast: using the Hihiirokane to cut through it, and then using Nirvana to retrieve her. Terumi felt uncomfortable as Hihiirokane was brought up, remembering how the sword had been used by Tomonori a few years prior.
Ten days before the Beast was due to appear in Ishana, Terumi was present in a meeting held by Nine that was presented at all the different organizations on the island. Terumi, as usual, did not really care for it and was swinging on his chair and yawning loudly. The details of the plan had been obscured partially by Nine, who did not include Hihiirokane, Celica, or the Beast's heart in her description as these details were only known to the heroes. The organizations were unlikely to believe these details and therefore unlikely to support the plan if they heard them.
On the evening of December 30th 2109, Terumi schemed with Kazuma to further manipulate Trinity. He allowed Kazuma to take over their body and manipulate Trinity into thinking that Terumi was going to force Kazuma to kill Nine. Their plan was a success, as Trinity was shaken to her core and could now be manipulated by the pair of them for their own plans. The following day, Terumi and Trinity waited near the Kaka Village for the appearance of the Black Beast. It appeared, having been successfully teleported there as a part of the plan devised by Hakumen. It was writing in pain and was unable to heal its wounds. Hakumen ordered that they decapitate its many heads. Terumi used Ouroboros and strangled one of the heads with his Nox, pulling so tightly that eventually he pulled the chains clean through, decapitating the head cleanly. The other heroes followed suit, and finally the Beast lay still.
Rachel appeared nearby with Celica and Nirvana. The heart of the Beast had been killed, and so the body began to rapidly deflate into the shape and size of a man similar in shape to Ragna. Hakumen walked forward and decapitated it in one swing of Ookami. The Black Beast was dead and the Dark War was finally over. Now the rest of Terumi's plans were to be put into motion. Terumi, Hakumen, Nine, Jubei, Valkenhayn, and Trinity were all granted the title Six Heroes because of their incredible bravery and influence in ending the war.
Months turned into years. Kazuma had managed to convince Trinity to undo the Mind Eater spell on Terumi and they invited her to the cauldron underneath Ishana. They told her to hide behind a pillar and invited Nine to join them. When Nine arrived, Kazuma was in control. He had the pair of them coiled in Ouroboros and allowed Terumi to take over. When Terumi was in control, he activated the Blue Grimoire to open up the cauldron. With Nine finally bound and hopeless, Terumi took immense pleasure in kicking her as many times and as hard as he possibly could in her head before finally knocking her unconscious. He then thanked Trinity and held her above the entrance to the cauldron before letting her plummet to her death. Little did Terumi know, Trinity had transferred her soul into Muchourin as she was falling; she was not dead in a technical sense, but Terumi believed he had killed her. He then threw Nine into the Boundary, finally killing the woman who had control over him for the last few years. After killing Nine, he went for her newborn daughter, Kokonoe. But before he could kill her, she was rescued by the Kaka clan. Although disappointed, Terumi was enjoying his freedom again, and went after Jubei and Hakumen, who were killing cauldrons across the world.
The OLN and Mage's Guild at this point had fused to become the Novus Orbis Librarium. Jubei and Hakumen were in the Asian branch of the NOL's Geological Survey, based in the western tip of Asia. Terumi went to this branch and to the control room above the cauldron. He saw Hakumen and Jubei there and, from his position, opened the cauldron up. They saw Terumi and he made his way down to them.
Filled with glee, Terumi asked if they heard the news. Neither of them knew what he was talking about so he enlightened them: the news about Nine and Trinity's death. Hakumen asked who killed them while Jubei, who had only just recently married Nine, stood there unable to move. Terumi didn't answer, saying instead all that was left was them, Valkenhayn, and Rachel. At that moment, he launched Ouroboros at them both. They dodged and Hakumen went to strike him Ookami, but Terumi was too quick. He launched two chains at Jubei, and then ran towards Hakumen, kicking him in the jaw with such a force that it would shatter the bones of a normal man. Hakumen regained his composure and slashed at Terumi, who had used barrier magic to block the blow. Ookami met Terumi's chest, but the wound was shallow as now that Ookami's core was removed, it was only at a mere fraction of its strength. On top of this, Terumi's regeneration was now abnormally high.
Jubei attempted to cut down Ouroboros but with Musashi's core removed and Ouroboros' still intact, the blade was unable to cut it. The cauldron was now fully open and Terumi grew stronger from not just the cauldron, but also the sheer hatred radiating from both men. He summoned energy from the cauldron that took the form of a large snake and it struck at both Jubei and Hakumen, but only the latter was struck. Terumi taunted Jubei, telling him that now both Tomonori and Nine were dead, he might as well join them to stop them from sleeping together. The fight continued until Jubei activated the Eye of Tomonori, a gift from Rachel that revealed Terumi's true form. Terumi cursed her name before Hakumen grabbed him. Hakumen told Jubei to close the cauldron and to allow him to sacrifice himself so long as he could kill Terumi. Hearing this, Terumi panicked, not wanting to lose his vessel nor his freedom which had craved for so long. Hakumen took Terumi and threw themselves into the cauldron as Jubei bitterly closed the cauldron behind them. Kazuma was killed by this as his body could not withstand the Boundary, and Terumi was sealed inside. 
Ikaruga Civil War
Lurking in the back of Hazama's mind, Terumi watched as he interacted with Meifang Lapislazuli, the Colonel of the Wings of Justice division of the NOL. Upon being assaulted, Terumi's spirit form lept forth from Hazama's body and began to strangle the Colonel, threatening to cut her to pieces and feed her to pigs should she continue act proudly. Both Terumi and Hazama soon left afterwards.  In the 11th Hierarchical City of Shinatsu, Terumi, in his spirit form, was asked by Meifang near its cauldron to help her turn into a Nox Nyctores. He agreed to help, noting that she had nearly died. Terumi ordered her begin hating everything; the world; and herself. The plan ultimately failed, and Terumi returned to Hazama and Phantom. 
December, 2199 had come again for Terumi, but he, Hazama, and Relius had resolved to end the looping of time. Thanks to Hazama's manipulation, Noel Vermillion saved Ragna from falling into the cauldron within the 13th Hierarchical City of Kagutsuchi alongside Nu-13. Terumi briefly took as the front of Hazama's body, and he demanded that Noel recognize him and his existence as she had become the new Eye of the Azure. The plan was a success - the time loops had ended and Terumi's existence was Observed. He threw Ouroboros towards Ragna, only for Tager to suddenly appear and block the attack. Having decided he had already had enough success, Terumi left. 
Once one of the Six Heroes who defeated the Black Beast, as well as the one who plunged Ragna into despair, he may seem impulsive and shallow-sighted, but is actually quite crafty and clear-minded. Cold and cruel, he truly enjoys "despair" and "destruction" from the bottom of his heart, and starts the Imperator's even worse "banquet of madness."
|Stage 3: Bang Shishigami|
Terumi: Huh? The hell's this? What's going on? Why am I here? No, that's not right.
Terumi: What the hell are YOU doing here?
Bang: You are...low-life scum.
Terumi: Whoa whoa, is that really the way to greet someone? Are you tryin' to pick a fight with me?
Bang: You're trash... Scum... A piece of crap! Stuuuuupid, baldy, rotten, fatty...!
Terumi: ... Alright, it's on, you shit! I have no idea what you're up to, but you pick a fight with me, and you're seriously gonna get it. You know what that means, right?
Terumi: Man... I barley felt a thing. It's like I was fighting air or something. Damn that felt weird. Though... I should've expected it. Argh, dammit... You're really outdone it now, hah! To try and take over a man's mind...!
|Stage 6: Ragna the Bloodedge|
Terumi: Oh, so now it's you, little Raggy? Chase me this far, you've really got some issues, boy. Now, tell me where you'd like me to start. I'll tear you apart limb from limb. I'm sorta good at killing people, you know?
Ragna: Killing people? Who are you kidding? You're just a reptile crawled out of some cold place in the earth. Put on some clothes, ya freak!
Terumi: You're riding my very last nerve, asshole. Quit barkin' or get ready to face some consequences.
Ragna: Shut up you piece of shit, you've given me enough lip. I could chop you up with one arm tied behind my back.
Terumi: Ooooooooh, I get it. You're trying to be clever, right? Riiiiiight? Still, this is effed up. How could that thing have this much power?
Ragna: Hey, I'm still over here when you're done talking to yourself. Just whenever you're ready.
Terumi: Keep up that barking, keep on pushin', see where it gets you with me. ...You know what. Fine, I'll play with you, but don't come crying to me if you're not in one piece after.
Terumi: Annnnnnd it's gone. This ain't fun. More importantly... This isn't some illusion or mind contamination witchcraft. This is real "materialization". Heh, someone who can materialize stuff...? You're kidding... ...
|Stage 7: Yuuki Terumi|
Terumi: Oh? Is this supposed to be tonight's entertainment? You've got some FINE taste!
Other Terumi: You're pretty talkative, now aren't ya? I'm not real sure what's going on, but this definitely looks interesting. I doubt that bitch can see this far... Why don't we enjoy ourselves a little bit, huh?
Terumi: Oh boy, I just felt a shiver go down my spine... Like when someone tries to show you an old photo album you just don't wanna see. Please, don't invite me to your reunion. I really am not in the mood to see people right now.
Other Terumi: If you got time to blab, you got time to fight. If you're scared, though, I don't blame you. Just run along home.
Terumi: Ugh, that shit-eating grin in his voice is just pissing me off.
Terumi: C'mon, c'mon! Hahahahahaha! Done so soon? Huh? If you got time to copy me, why don't you improve your style a little, too? Hurts my feelings to have SUCH a weak clone. You're still here. And I don't have that strange feeling from before. Which can only mean... Oh, what an annoying little bitch.
|Stage 8: Hakumen|
Terumi: Haven't you had enough yet, asshole? I don't have time to play games all day.
Terumi: I knew it was you, glasses... Materializing someone inside my mind...you sure've learned a few new tricks.
Trinity: So you've noticed... Terumi.
Terumi: Obviously. Who do you think made adjustments to your Muchorin, huh? But one thing I can't figure out... Your Muchorin and you alone couldn't pull something like this off. Which must mean, the vampire's behind all this. So I'm right... Damn bloodsucker.
Trinity: ...Mr. Terumi. I've come here to defeat you. In order to...obliterate you.
Terumi: Obliterate? Wait wait wait... You mean YOU? Obliterate ME? Now that's just cold, Trinity. We WERE on the same side, you know?
Trinity: The same side... And who was responsible for tearing us all apart?
Terumi: Aw, what's this, what's this? You're the one who was crying from joy or whatever. So EAGER to help me, right? "If there's anything that I can do..." something or other. Which would really make us both...accomplices. You're just as much a traitor as I am!
Trinity: Yes, that may be so. I am a traitor, like you. To those who are dear to me, those I should never betray. Which is why I must chase you. In order to kill one more stem my foolishness has nurtured. The next person I will materialize... Is the strongest one I know! That will be...the consummation of my thoughts against you.
Terumi: The strongest, huh? Heh heh heh... You're really pulling out the big guns this time, aren't you?
Hakumen: Sin redeemed by sin, punishment by punishment. It is the time of reckoning, Terumi!
Terumi: *Pant pant pant* Huh!?
Terumi: Dammit, I went all out cuz I was against you, Hakky... But it turns out you weren't all that. Can't shake off the old habits, hm? You really need to improve that imagination of yours, Trinity. Cuz the Hakky I knew wasn't this WEAK!
Terumi: I suppose that's all you can do, though. You should really learn your place, and keep your nose out of my business. Hm? Or I'll give you another vacation in the Boundary. Heh heh heh heh... HAHAHAHAHA!
Trinity: It is just as you say. In the end, this is all I am capable of. I cannot fight alone. All I do is help others do the fighting. ...I can do right now, is... ...to make it as easy as possible for someone to stick a knife... ...through your THROAT!
Terumi: ...Hey, what the hell are you...?
Trinity: I should've done this a long time ago. For my dear friends...who meant the world to me.
Terumi: Tch, what the hell are you rambling about? You're not even making sense anymore... Huh?
Terumi: This is... No, not in a dream anymore. This is the real... Ikaruga? Dammit, what the hell's going on? What the hell am I doing in Ikaruga!?
Hazama: Um, hello? Can you hear me? It appears we've switched positions, now hasn't it...? I used to be "heads" and you, "tails". But now you're the "head"... Or something like that?
Terumi: Whoa... Whoa whoa whoa, wait a second. You've got to be kidding me. This means... This means that damn cat's sword can reach me now...!
Hazama: Oh, that sounds pretty bad... What to do...?
Terumi: God damn both of them, the bitch in glasses and the vampire!
Terumi was given a mission by Izanami – to secure multiple areas across the Ikaruga Region in order to erect a barrier that would be used in Doomsday. He returned to Ishana and reported his progress to Izanami before aggressively approaching her and asking why she allowed Ragna to escape back in Kagutsuchi. The thin man backed down when she told him that she could easily end his life. As the Imperator left, he remarked that it had been a while since he was in this part of the world; he had not been in this part of Ishana since his fusion with Kazuma and the murder of Tomonori. Terumi then activated the Blue Grimoire and began laughing in hysterics, wondering what possibility Amaterasu would show him.
Both Hazama and Terumi entered the Realm, a subconscious and imaginary space created by Noel. He saw that she had accepted her power as Mu, acknowledging that she could kill Gods with her power, but remarking that power is nothing more than what someone seeks when they want superiority over others. The men had Nu fight against Noel, leaving when the Realm collapsed. Back in Ishana, Terumi expressed his glee at finally getting revenge on Amaterasu.
Terumi and Hazama made their way to the Colosseum to see the events for themselves, finding Ragna in one of its entrances. They battled, winning against the Grim Reaper – Terumi took solace in his victory and cruelly kicked Ragna as hard as he could repeatedly. He then dodged a surprise attack from Tager, turning and seeing both Kokonoe and Celica. Terumi was shocked at Celica's appearance, knowing he had killed her years ago; Phantom appeared and teleported him out of there as Terumi screamed at Kokonoe, calling her the devil. When he arrived back in Ishana, he screamed at Relius for not telling about Celica's existence, promising the scientist that he was going to kill him one day. He watched as Izanami ordered Phantom to destroy Takamagahara.
Terumi had a body materialized by Trinity against his wishes and he was susceptible to being killed. Under Hazama's orders he went to the Monolith near Ibukido where he was found by Hakumen. The two battled, Terumi determined to have as much fun as possible and suggesting that he take back his original body. Hazama, however, was mortally wounded by Trinity, and the link that Terumi and Hazama share made the God fall, giving Hakumen enough time to land Time Killer, which killed Terumi by stripping away all his remaining time in the world. Fortunately for him, this was not the end, as he begun to self-Observe.
After being forced against an onslaught of materialized battles against Bang, Ragna, Valkenhayn, Jubei, Hakumen, and Trinity, Terumi's body was materialized in the real world by Trinity, who entered his mind. He found himself in Ikaruga, and was found by the real Jubei, Hakumen, and Celica. He lost to the knight but was saved by Phantom, who teleported him away as soon as she could.
Once one of the Six Heroes who defeated the Black Beast. When he saw the transformed world, he muttered "Gotta redo it again?" and began to act in order to bring about the same history as before. His goal is twofold: to destroy the Master Unit, and to obtain a weapon of his own....
Terumi barely survived his encounter with Hakumen. The body Trinity made for him died, but he was able to survive through crudely Observing himself, although this method would only keep him sustained for a week at most. He began searching for another vessel.
He found Hazama, explained what had happened, and through him, ordered that Noel go to the Kaka Village to deal with Ragna.  He stayed with Hazama, barely being able to possess him, and watched as Nu battled against both Ragna and Noel.  Not long afterwards, he was transported to Ikaruga, and told by Nine that he could obtain the Azure by killing Izanami. Although Hazama had told him that he would search for the Azure himself so they could fuse, Terumi began to search for a different vessel to fuse with, finding that he had outgrown him.  Hazama had fulfilled his side of the deal, however, and had gained an Azure piece from Izanami – he told Terumi he wanted to fuse, but to be in control, and he complied. 
Going to the Alucard Castle, Terumi pried open Clavis' coffin after battling through the Prototype Absolute Defense : Blade of Reflections, which created a perfect copy of himself to fight. Winning, he took the Hihiirokane within,  and used to separate himself permanently from Hazama when his former vessel tried to consume him. Terumi then absorbed the weapon into himself and left Hazama to his own devices.  He drove Nu insane after she was defeated by Ragna, forcing her to fight Hakumen. While the knight was distracted, Terumi launched Hihiirokane at him, shattering his soul and taking over the Susano'o Unit, finally becoming Susano'o once more. He beat up everyone present there effortlessly then snatched Noel and Amaterasu, forcing them outside of the Embryo. 
Terumi exited and returned to the unit frequently, being able to now summon it at will. At the gate with Noel in his grasp, Terumi battled against Es, the protector using the Susano'o Unit to do so. Amaterasu had Observed his victory before the battle even began.  Soon Ragna and Jin arrived, and through their combined efforts, sent Terumi out of the Susano'o Unit. He was sent inside the Gate with Ragna by Noel, coming to the Azure Horizon.  He fought his nemesis for the True Azure, and lost, finally dying after having cheated death countless times. 
- BlazBlue: Phase 0, Chapter 1 - Lost White
- BlazBlue: Phase 0, Chapter 5 - Sealed Green
- While it is never outright stated that Terumi was self-Observing, it is the only feasible way he could have appeared before Kazuma
- BlazBlue: Phase Shift 1, Chapter 2 - Cat of Crimson Sunset
- BlazBlue: Phase Shift 1, Chapter 3 - Mind of Azure Deviation
- BlazBlue: Phase Shift 1, Chapter 4 - Doubt of Yellow Equilibrium
- BlazBlue: Phase Shift 1, Chapter 6 - Voice of Emerald Evil
- BlazBlue: Phase Shift 1, Prologue
- BlazBlue: Phase Shift 2, Chapter 1 - White Blade, A Flower
- BlazBlue: Phase Shift 2, Chapter 2 - Purple Ones, Their Theory
- BlazBlue: Phase Shift 2, Chapter 3 - Scarlet Guardian, the Doll
- BlazBlue: Phase Shift 2, Chapter 4 - Indigo Heart, the Conception
- BlazBlue: Phase Shift 2, Chapter 5 - Black Beast, Reappears
- BlazBlue: Phase Shift 3, Chapter 1 - Battlefield with Blue Sky
- BlazBlue: Phase Shift 3, Chapter 2 – Turbulent Ashen Current
- BlazBlue: Phase Shift 3, Chapter 3 – Copper Catgirl
- BlazBlue: Phase Shift 3, Chapter 4 – Silvery Illusion
- BlazBlue: Phase Shift 3, Chapter 5 – Hope and Despair on the Green Hilltop
- BlazBlue: Phase Shift 4, Chapter 2 – Sweeping Away Crimson Grief
- BlazBlue: Phase Shift 4, Chapter 3 – Demonic Human Weapon
- BlazBlue: Phase Shift 4, Chapter 4 – Azure Wish's Fate
- BlazBlue: Phase Shift 4, Chapter – Colorless Guiding Light
- BlazBlue: Phase Shift 4, Chapter 5 – Blue Snake's Crazed Play
- BlazBlue Material Collection, Iron Tager Short Story - Trust You
- BlazBlue: Variable Heart, Chapter 04
- BlazBlue: Variable Heart, Chapter 18
- BlazBlue: Calamity Trigger - Part 2, Chapter 5 - Calamity Trigger/Successor to the Azure
- BlazBlue: Chrono Phantasma Official Site (JP), Yuuki Terumi
- BlazBlue: Chrono Phantasma, Story Mode, Prologue
- BlazBlue: Chrono Phantasma, Story Mode, Chrono Phantasma, Episode 12: Uninvited Guest
- BlazBlue: Chrono Phantasma, Story Mode, Chrono Phantasma, Episode 13: Mad Dog
- BlazBlue: Chrono Phantasma, Story Mode, Chrono Phantasma, Episode 15: The Truth
- BlazBlue: Chrono Phantasma, Story Mode, Chrono Phantasma, Episode 16: Kushinada's Lynchpin
- BlazBlue: Chrono Phantasma, Story Mode, Episode 18: Beginning of the End
- BlazBlue: Chrono Phantasma, Story Mode, Six Heroes, Episode 8: Bonds of Fate
- BlazBlue: Central Fiction Consumer Edition Official Site (JP), Story - Yuuki Terumi
- BlazBlue: Central Fiction, Episode 003, Chapter 016
- BlazBlue: Central Fiction, Story Mode, Episode 004, Chapter 033
- BlazBlue: Central Fiction, Story Mode, Episode 006, Chapter 043
- BlazBlue: Central Fiction, Story Mode, Episode 010, Chapter 079, Sub Scenario 1
- BlazBlue: Central Fiction, Arcade Mode, Yuuki Terumi, Act III
- BlazBlue: Central Fiction, Story Mode, Episode 010, Chapter 086, Sub Scenario 1
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 092
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 093
- BlazBlue: Central Fiction, Story Mode, Episode 012, Chapter 096
- BlazBlue: Central Fiction, Story Mode, Episode 012, Chapter 097
|Other (C-series)||Edgar and Claire Vermillion ·Ellen Nelson · Four · Francis and Rosenne · Lao Jiu · Pakumen & Terumen · Three · Two|
|Remix Heart||Elina · Guardian ·|
|Variable Heart||Julian ·|
|Alternative: Dark War|
|Persona 4 Arena/
|Under Night In-Birth|