That aside... Man, is there a single redeemable character beside the main couple? The oneesan is the worst kind of scumbag, her friend is also a scumbag, Tsukimi's friends are scumbags, MC's friend is a bastard, the trio of Tsukimi's "fans" are creeps... Now a dad is also, unless it's some kind of tough act, a bastard... Seriously, i bet if teacher had more screen time than 2-3 panels here and there, she would probably also get revealed as a bitch =_=
Cat isn't a scumbag, she's just a cat. Prickly but loyal, playful but dangerous. Once she realized that Rabbit is, well, a Rabbit, her whole demeanor changed, and even got flustered when she got saved.
The humans are pretty common manga mobs, but that's to be expected. I get her friends, too, they're pretty common characters. They roll their eyes at the guy their friend likes, but they want him to be good for her and are annoyed if he isn't.
All told, I don't think the manga's full of unlikable people. The ones who you don't like are the ones you're not
supposed to, because they're the antagonists. The sister and the main girl's male admirers are the obstacles standing in the way of their relationship, and you're not supposed to like those.