[Eater of All]

Originally Posted by Haak

I'm sure that will come up, but Erii is definitely important to Uiharu, and Uiharu has never had the oppurtunity to the 'big sister' type to anyone, and she clearly wants to. You already know that obviously, but my point is that I don't think this is a situation Kuroko and Uiharu have been in before, nor do I think they've been in any similar situation so naturally there would be complications.

Such a conflict can serve to strengthen their bond, I guess. Uiharu's blind support for Haruue against her friends still annoys the heck out of me though, especially given that she barely met the girl less than a month ago.

Originally Posted by Wilfriback

If they dissapear then they aren't main characters, just support/side characters.
BTW, I wonder what kind of concept pf "development" have the people in mind, far I have seen its just slice of life doing things that would make any day, you can skip them and the characters would be the same.

I know this is rehashed alot, but the least Index can do is give its title character more screentime than an average antagonist of an arc. Splitting the story into distinct arcs means we're stuck with Touma as the only reoccurring character of the whole series, and there's only so much one character can do before it starts to get tedious.
As for development thing, I assume nowadays it's more used as "fleshing out a character" rather than solely "having characters undergo changes." Your aforementioned slice of life stuff fits the bill for the former, and looking back at the end of the Level Upper arc, you can see that Saten and Mikoto also changed a fair bit as well.