Their relationship will blossom in a way Arima doesn't expect. He is smart for a kid his age (even if stupid) and this ultimately makes him paternal when a girl he knows constantly gets herself into trouble.
This type of dependency a gal builds can be INSANELY intoxicating for paternal boys/men (I know from experience). He'll end up seeing her as his escape from stress because her problems are absolutely nothing. He'll feel powerful when he opens a pickle jar because she is so happy when he does.