It feels like either Rize's brother was broken insane (looks like abuse to me) and he must've tried to do something to Kairi to the point that Kairi killed him on self-defense, either that the brother killed himself and Kairi didn't try to save him. Any way you look at it, Rize's brother had been abused and I can tell u from personal experience, that kind of childhood abuse breaks a person and renders them insane. If it's anything but this then that's a plot twist I'm looking forward to.