Koharu is really interesting, actually. Sumire is the only person like her. She's probably overjoyed at the idea of finding someone else who can do the things she can, and has been convinced that she's saving her from being exploited and hurt. She doesn't understand the relationships here, but doesn't feel she needs to. Because she's sure she understands Sumire. How could she not? They're both the only ones who have the powers they do. Sumire's just like her.
I really, really hope she survives. I like her and I actually want the two to become friends. She's insane, but genuinely seems to care about Sumire in her own weird way.