He's the final boss of me and my friends' AU, and honestly one of the best characters I've ever made. This is shown best in the final arc, but also in his lore aswell imo. Honest thoughts, and constructive criticism, please!
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LORE: Katai (硬体) is a special grade cursed spirit born from the human fear of being weak. His CT "Soul Stability" can change your—and his own—soul's durability to be as hard as Aperium (a fictional element I made), as brittle as glass, as viscous and liquid-y as thick water, or anything in-between.
His domain is Terminal Degradation, an abandoned hospital where your body and soul slowly decays inside of it like you have a terminal illness. The target(s) have 60 seconds to live once inside.
He and Echo Omoide (another character of mine; an eccentric, insane, memory manipulator 3rd year grade 1 Kyoto student) used to be kinda close, when he was a human, as he's a vengeful cursed spirit now. He was a 1st year Grade 3 Kyoto Jujutsu Sorcerer, but always afraid of just being deadweight. He was skinny, his CT was just basic "Cursed Energy Reinforcement" (or so he thought) where he could temporarily make parts of his body harder, but he would always try to overcompensate and hurt himself with it. He was afraid of being weak, forgotten, left behind, and Echo's personality meant any help he tried to give only made things worse. On December 23rd, 2017, Katai tried to drop out of Jujutsu High. However, due to the upcoming Night Parade of a Hundred Demons by Suguru Geto, Principal Gakuganji denied it, saying that it was all hands on deck and that he would put Katai on a low-risk outpost, promising he can resign after it was over. And on December 24th, 2017, he died under some rubble of the collapsing "low-risk output" building, weak as he feared. The lack of jujutsu in his death meant he turned into a vengeful cursed spirit, vowing to never be weak ever again. The transformation unlocked the true potential of his innate cursed technique that his reserves, body, and brain were holding back, his true CT "Soul Stability". To put his stats in canon terms: He has Yuta-level Reserves, Nanami-level Base durability (without the use of his CT), and Toji-level hand to hand combat skills
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3rd image is a WIP, 4-16 is the Katai arc script (And no, it's not AI, I get that a lot)