Relationships
Actually this section should just be about Naota, because I could go on and on about Ninamori's relationship with Naota. But I felt obligated to throw Gaku, Masashi, and Mamimi in as well. :P

nandaba naota - secret crush and secret understanding
I think it's pretty obvious that Ninamori has a crush on Naota. She has a preoccupation with him: picking on him, ordering him around as the class president for the school play that she just happened to rig so that he would play a cat, and who could forget the fuzzy lovey shoujo-type-anime moment when Haruko hit Naota with her bike and he flew at a high speed towards Ninamori and almost kissed her? She likes him.
The only time she's really overt about it though is when she spends the night at Naota's house during her family crisis and she confesses to him that she rigged the votes for the play and how she thinks he's perfect for the part of the cat, and then kind of starts hitting on (cornering? attacking?) him. When he doesn't return her feelings though, or understand at all what she's saying to him, she seems to lose interest in him "romantically"..
But why does Ninamori like him? Maybe because she sees that they have the same problem about wearing false faces for other people. Maybe she wants to have him help her to be better at her little pretender act, since he knows so much about it, which is why she cast him as the cat. Or, maybe she can just identify with him.
More important than that, though, is the fact that she understands Naota. She sees stuff about Naota that no one else seems to notice or care about (except for maybe Mamimi) and that even he himself is oblivious to. She cast him as the cat in the play because the cat is a liar, and she knows Naota's got a false face on half the time just like she does. She can really relate to the guy. Unfortunately, seeing as he's a twelve year old boy, he doesn't seem to notice that about her at all. Well, he doesn't even notice it about himself. Unlike Ninamori, whose act is a completely conscious decision, Naota's doesn't always seem to be. Ninamori is much more self-aware than he is.
Anyway, it might just be me, but in episodes five and six, after Ninamori resolves her family issues and Naota's really starting to lay on the fake cool factor and then crash down from it after Haruko leaves - doesn't it seem like every look she gives him just seems so knowing? Or wise? Like she knows exactly what Naota's going through (because she's just been through it too).
He doesn't seem to think much of her though. He calls her a brat and a show-off. But then, she did try to blackmail him into acting in the play by bringing up the fact that he has an abnormal growth of cat ears sprouting on his head. So you can't really blame him.

gaku & masashi - just classmates
I thought Fatlips and Glasses were important to throw in because Ninamori's always seen with them. With that being said, though, she doesn't really have any obvious connection with them. She's with them a lot, but they're just.. there. I don't know, it might just be because Gaku and Masashi aren't really well developed or important characters, so their relationship with Ninamori isn't well developed or important either.
The only really distinguishable interaction I can mention is in Episode 2 - Gaku, Musashi, Naota, and Ninamori are checking out the boathouse that's been burned down. They walk up to the burned out building, which is taped off, and Ninamori mentions that it says "do not enter." Glasses says back kind of nasty, "Then don't follow us, Ninamori." 6th grade boys are not the nicest breed, especially when talking to 6th grade girls, so I'm not sure how much I should read into this - it could just be typifying preteen behavior. But I'm thinking maybe Ninamori is just tagging along with them (because of Naota, maybe?), and they don't particularly want her there. After all, Glasses only called her by her surname, as if he didn't know her well, and he sounded kind of snarky when he did it. Are these two just the classmates she would like to get close to at this point, but can't?
Later on, after episode three, she accompanies these two on their job driving a truck near the river, and also reveals to them during gym class that Naota should just say how he really feels, because it's what she did herself. So I'm thinking it's kind of likely that she and the boys did actually end up friends after she realized she was just a kid after all. She didn't end up transferring out of her school, either, and that could be why.
I think Gaku & Musashi are just supposed to be regular kids. When Ninamori tries to act all grown up, she can't really fit in with them. And when Naota really starts acting ridiculous (after Ninamori has conveniently gotten over her adult syndrome) he doesn't hang out with them like he used to, either. (So Ninamori does instead.) But, in one of the very last scenes in FLCL, all four of them are standing together on their first day of high school, so.. I guess they all ended up as one big happy preteen clique after all.

samejima mamimi - a former hapless victim
Mamimi is at the opposite end of the spectrum from Ninamori. Mamimi doesn't care at all about how she appears to other people and acts like a perpetual child most of the time.. and Ninamori doesn't think too highly of her because of it, and insults her for her openly childish behavior. She thinks Mamimi is stupid. (By the way I'm not trying to put Mamimi down or anything by saying all of this - she's actually my favorite character - but you have to admit, the girl is weird).
However, in episode five, after Ninamori resolves her problems she happens to see Mamimi and doesn't say anything about her. Granted, Mamimi's not hopping all over Naota, playing a video game, or walking around with lighters taped to her forehead at this point, but still, it kind of points to the fact that Ninamori has grown up a little, right?
By the way, these two never actually have a conversation. I don't even think Mamimi is aware that Ninamori Eri exists. Whenever Ninamori talks about her, it's never to her face.
x