@Grahav
The difference between Minami and Tomoko is that no matter how much really horrible shit Tomoko thinks regarding people, and spite at some things that may happen to them, she NEVER aggressively tries to make life bad for them, socially, and she never says most of the things she thinks. Like Minami, the things Tomoko actually DID came back at her, like cheating those kids.
That's why people come back so hard at Minami. She's a mean, malicious little shit. Their slapping her back is a survival instinct; if she says something bad about someone, she'll say it about everyone else. She's the kind of kid who will crack up laughing if someone faceplants on a concrete floor. It's quite obvious from the outside that she does what she does, namely discount people's importance based upon relative social worth and the likelihood she'll have to deal with them again.
On the other hand, she is kind of pathetic because she really does suspect that "no one gives a damn about me", I suspect, as she said at Disneyland. Asuka, from the first time we saw them together, has had a marked distaste to the idea of her being around, and Akane turned out to have simply treated her as a dingleberry, and Mako acts as Minami's therapist.
I think there's more to her coming in the story. She has just been too carefully drawn for it not to be so. I mean, it took centuries with Uchi, but her development finally happened and she is a fully realized dynamic character now.