Okay then this will be more.detailed.
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So the brother already has a deep obsession with Runnan. It's because of well war trauma that he himself has and so he wants runnan to stay safe and love him the most, because that's how it was when she was just a child and loved her brother before he was sent off and well got traumatized.
In the eyes of the parents he's the shinning older brother who is very talented and stuff like that and he also seemed the same to runnan until he realised that Runnan had a pet squirrel she was very fond of, so he killed it because he wanted runnan to love him the most.
It was because of that thing that Runnan became terrified of her brother who would go to any lengths to be the person she loves the most and that includes killing anyone else she might consider more important or tries to even harm her like even in a playful match.
The family is obviously unaware of it and love both their kids the same because he only shows that side of himself to runnan and because she's so scared of him hurting others and people she loves she ended turning her emotions off.
So she wasn't always this Spacey person it's like a emotional switch off because if she doesn't care about others or get close to others he wouldn't hurt anyone else
Of course Raon is going to help her get over it with her own powers because Raon is a good guy and he will always look out for his family.
Almost forgot this bastard existed like .... Fuck offfffffff