Of course, it's only a coincidence that Naota's name happens to contain the syllable "ta"... Ta-kun is either what she called Tasuku, or what she would have called Tasuku if he had in fact been her boyfriend (though something tells me that since he wasn't, she never actually called him Ta-kun to his face). The fact that she calls Naota (and anything else she's affectionate toward) Ta-kun just reinforces how obsessed and delusional she is.
Of course, her feelings for Tasuku are a form of hero worship. He's obviously older, and as such probably a bit intimidating to her as far as how much familiarity and affection she could show him and still keep him around. She makes up for this lack of showing affection after he leaves by giving the nickname that would have been his to his brother, upon whom she bestows her pent-up sexual frustration.
What I find interesting is Naota's attitude toward this relationship. He obviously knows that any affection Mamimi shows him is just displaced affection for his brother, and not only does he not mind, he seems to think that maybe if he abides it for long enough, Mamimi will gain some real affection for him. The problem is that Mamimi's "affection" for Tasuku is mostly just sexual, and because of Naota's age, her projection of Tasuku onto him cannot be completed in this way. Mamimi seems willing to use Naota only up to a certain point (because she doesn't see him as a real prospect, just like Tasuku didn't see her as a real prospect), and only in ways that she would have treated Tasuku.
Mamimi does respect Naota, but only as a substitute for his brother, not as a person in his own right. I believe this is the other part of the revelation she gets--that Naota is not Tasuku, and Tasuku is not her boyfriend. She only realizes this, though, when she is forced by Naota's actions (especially the whole saving-the-town thing) to see him as a real person and not just some sort of Tasuku-doll.
...Dammit, why can't I be this insightful about stuff I'm *supposed* to be writing?
Since Naota isn't willing to accept that he is a child, he isn't willing to accept anything that goes along with growing up, like adolescence. So, although Mamimi's advances are sexual in nature, to Naota's "grown-up" mind, it's real affection. I think this is why he says he hates Haruko all the time.