Yeah but Kate is not your mum
You sure about that?
The MC doesn't know who his parents are. His grandfather lied to him about his background, so it's entirely possible his mother is still alive. The MC explained to Kate that if pure demon males have male offspring with pure demon females like Kate, the male offspring take after the father except they're far stronger, which is exactly the case with the MC (the blond demons explicitly comment on his power). Hannah and the MC are "about" the same age, so Kate could easily be his mother/Hannah could be his twin.
The MC's grandfather also referred to Kate's mother as his best friend, and (in the most recent of the MC's flashbacks) said that he lost his best friend during the fight with the MC's father; why was Kate's mother involved in that fight, if it's not related to Kate at all?