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.
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 Ikuraga 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 Defence: 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 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 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