I thought that Aeris was one of the least interesting characters. Horribly cliche once you think about it. Oh she's the last of her kind, adopted into the middle of the slums, where her house is the only one that's remotely nice (allegory to her being intrinsically "good"). Of course everyone is after her because she's special in some way. She encounters the protagonist by chance, and he winds up trying to protect/escort her to where she has to go. Bad guy kills her (OK, slight deviation), but she ultimately winds up being the major proponent to saving the day.

Ultimately, Aeris is just that really, really nice girl who everyone pretty much likes because she's so nice. Then she dies. It's tragic, yes, but her niceness doesn't make her interesting. Her people might have an interesting background, but it's not at all a tale of survival like Nanaki and his parents deaths. Now THAT is interesting stuff.

To be fair, she becomes more interesting after her death, but I don't really consider that all that fair. She was largely bland compared to the other characters.