Just discovered you today, I love your channel 🙏🏾

Haven't read ahead in the manga, but pretty sure that Kiruko's thing is the brother's body was beyond saving from the creature so he couldn't be saved in his current body, being half gone. She, in her grief and in the moment, took her own life with his 'gun'. Shot to the head, leaving her brain destroyed and his body destroyed. Thats why the doctor transferred his brain into her body. I'm pretty sure thats what happened with how their personalities got changed. Also why its only his mind, and you don't have things like her mind trapped in there as well or something. She is gone-gone, and he is literally now trapped in a womans body with his brain still wired as it was.