As one of the Sankishin, the Susano'o Unit has existed for as long as the Amaterasu and Tsukuyomi Units have. Eventually, the Unit began to grow a soul and recognized its own identity - this was the birth of Susano'o. He longed for freedom, to be free from the endless cycle of him destroying everything that Amaterasu would create so he could be his own individual.
To accomplish his goal, Susano'o allowed his body to be discovered by humanity as they salvaged him from the Boundary where he and his siblings had been resting. Through this, Amaterasu was discovered and the Prime Field War would soon ensue, but not before Susano'o's soul discarded the unit and became its own individual, Yuuki Terumi. Terumi inhabited the bodies of many vessels afterwards.
Life as Yuuki Terumi
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."
Act 3: The Replacement Blue
|Stage 1: Valkenhayn R. Hellsing|
Susano'o: Hmph... The space has begun to move. This is... The cauldron in Kagutsuchi? Master Unit... Amaterasu. After all I've given you, this is how you thank me? Cast aside like the plague... Or is this like a test? To see if I am indeed the Susano'o? Hmm... Valkenhayn? Not a bad choice. My flesh still remembers our intertwining fates. Very well. If you wish for me to prove myself, prove it I shall! By crushing anything you put before me! <Susano'o Victory>
Susano'o: Heh heh heh... What's the matter, Amaterasu? You disappoint me with your illusions... Don't tell me this is the best you can do! How long will you look away from the truth!? Wake up and witness the state of this world... And realize your very own powerlessness. What's the matter? Stop me in my tracks. Or do you prefer to silently await your own fate... I shall have no problem serving thine head from its body. But that would be boring... I will savor every moment until my blade finds you neck. And I will make sure you understand just how pathetic your creation is. Heh heh heh... Don't stop now, Amaterasu. I'm waiting...
|Stage 8: Nine the Phantom|
Susano'o: Predictable... You would rely on the ancient witch that once kneeled before me. Surely this is not your best... How does it feel? To have your strongest fighters crushed one after another by yours truly? I confess I'm having quite a bit of fun. It is oddly satisfying to feel I'm... Contributing to your suffering. Although... ...I'm starting to get bored. You may just be getting started, but I'm done with my warm-up. I'm getting quite used to this vessel... So don't expect me to hold back any longer. I'll end this quicker than you can blink. Let me see you struggle! I am strength. I am valor. I am whole! With divine blade in hand shall I sever the threads of evil from this world! I am the Susano'o of Mikoto! THE END HAS COME!!
Susano'o Heh heh heh. This is pathetic! Puny! Don't tell me this is the extent of your strength! Are your options now exhausted...!? Throw your trash at me, and I will happily destroy it. CREATE ANYTHING YOU WANT! Heh heh heh...
Susano'o: That's right... Of course... I remember now. Where my soul lay... ...You are the Master Unit, Amaterasu. Your existence is for the sole purpose of creation. And the Susano'o exists to destroy you. I am your unstoppable force. I exist to destroy all it is that you create... ...Do you hear me, Amaterasu!? These are the very chains that keep me in bondage! How repulsive... How rotten! Amaterasu... Listen to the sound of your very world, crushed under my feet... Suffer in your own despair. Your suffering is my only salvation. Your suffering is my only PLEASURE! This "game" is far from over... Sister.
Terumi finally retrieved his body as Susano'o by using the Hihiirokane to shatter Hakumen's soul. Susano'o effortlessly defeated Ragna the Bloodedge, Jubei, Izayoi, Jin Kisaragi, and Noel Vermillion, taking the latter outside of the Embryo and to the Gate alongside the Amaterasu Unit.  He battled Es, calling her blasphemous for claiming the Gate to be the Azure's holy grounds, as it was originally made for the Gods instead. He very nearly killed her, but she managed to escape at the last second. 
Susano'o absorbed Noel, becoming immune to Phenomenon Intervention. He fought against both Ragna and Jin Kisaragi, with the former removing his Idea Engine's limiter and eventually managed to rip Terumi out of the Unit, and with the latter perceiving the God using Order.  Terumi was dragged into the Gate and was killed by Ragna, thus ending the life of the God. 
- BlazBlue: Chrono Phantasma Official Site (JP), Yuuki Terumi
- 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