Aya. There are many reasons I don't like Aya. I'll go over those reasons:
I don't usually like idols. No particular reason for that. I just don't like them.
I don't like female characters that are meant to be all girly, cute, and bubbly. They seem too stereotypical to me. But if a male character has those traits, I immediately love them.
I don't like her voice. It's just kind of annoying to me.
Of course, though, this is just my opinion. The traits that I don't like about her might be the traits that other people do like about her.