i think the recent chapter says that the body MC is in was adopted into the girl's family. and it's weird how they sometimes filter their dynamic, but i do ultimately think that the girl feels only sibling love towards the original body, and treats MC the same way. and MC, despite not really sharing any emotional or sibling bond towards her, definitely has some type of emotional loyalty to them. i just think that the author is just being fucking weird with their undertones for the sake of fanservice.

So what's his relationship with that girl? Brother-sister? Or is he just happened to live in the same roof but not blood related? I swear to God, if they're brother and sister I'm going to drop this.