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Despite that Yakushimaru/Myoe tends to be a jerk to Koto, he does care for her particularly in episodes 8 and 9 where he managed to bring Koto back to her senses twice when she blamed herself for Mirror Kyoto's destruction.
Prior to that, when Koto enters his room and vents her frustrations for wanting to know about herself, Myoe was about to comfort her until Inari arrives.
Somehow, Myoe understands Koto's predicament since he lost his original family and is willing to help her know more about her parents. It's understandable that he's very angry at Inari for not telling Koto about her past which forced her to go to Mirror Kyoto, resulting in the city's destruction and then, putting all the blame on her.
The little pep-talks Myoe gets from his older siblings in Episode 9.
Fear not, Myoe. The things you ought to do are not in the past just yet. Look ahead. Move forward. Don't worry. Your past has not disappeared. Desiring a future does not betray the past, Yakushimaru.
But you don't think this place is so bad, right? You always wished for your own death, but you never once wished for the Mirror Capital's. So even though you've always been quite hopeless as a little brother, I always felt you'd make a fine Myoe.