This is what I think, Kuroha has a bad hidden motive, Kachi likes Maru (obviously) and she's actually his childhood friend, Abe senpai likes Kuroha so he and Kachi pretend to be a couple. It's all part of their plan?
Lemme guess, the long haired girl is another childhood friend of him hence the "suu-chan" and the short haired girl is maybee slightly manipulative and I'm not sure if she's good or a bad person.