Mari seems to be a girl of extremes. Extreme devotion to her boyfriend, extreme depression when she discovered he was cheating on her, extreme obsession with Furuka when Furuka was even the tiniest bit nice to her... While, of course, around here we tend to approve of a girl taking a lesbian turn, her behavior really seems unhealthy. Especially since Furuka isn't even noticing her very unsubtle hints at all.
In other news, I really hope the chapter of Kon dissolving away was symbolic rather than literal, and she's actually still alive for Furuka to save.