To be honest, I don't know what secrets Kachi is hiding herself, but so far it's definitely Kuro making the worse impression. The audience is encouraged to root for her due to how the manga is framed, but if you try to look at things objectively her manipulative behaviour and how she gets in the way of what Maru seems to want does not reflect very kindly on her. Of course in normal circumstances, after finding out that Kachi has a boyfriend it would be the logical thing to do to support his moving on since he'd most likely have no chance, but seeing as it's been shown that something is off with Kachi and Kuro might know a thing or two about that, the way she tries to keep Maru away from her (preventing him from finding out what the deal is with her and if he really has any chances) seems to be for her own sake. And that's not something for a teenager honestly, but it's not going to make me like it any more.
I'm not saying that the other two don't have flaws either. At best, Maru is easily swayed and dense while Kachi is hesitant to act and giving mixed signals. But so far, Kachi has been the most annoying to me, because she's the one actively trying to prevent what her friend wants, because she wants him for herself while the others' flaws mostly harm themselves or at most unintentionally harm others. And I'm also not saying I support the Kachi ship either. The story shall flow as it flows and it's too early for me to have an opinion on how it will go (except pointing to the obvious, if it's that easy) or what I'd personally prefer. So, we'll see.
Future reveals could of course change everything, because perspective matters a lot, but this is just how things look right now.