This one's kind of tricky for me, because it'd be SO easy to name any one of my favorite idols. I'm going to say that this means character as in on-screen personality, and not actual character (i.e., not Charmy Ishikawa or Lovely Takahashi, actual separate characters).
With the given fact that any of my favorite idols (Ishikawa Rika, Oota Aika, Iwasa Misaki, Sasaki Ayaka etc.) would probably take this spot if I wasn't trying to be comprehensive about things, a character I like is...
AKB48's Sashihara Rino.
If you follow AKB then you're probably very familiar with Sashiko's antics, but if you're not, Sasshi is a wota. Big time. She followed AKB a lot before she joined, does wotagei, and talks about idols. A lot. Sashiko has been growing in popularity, and I imagine that her wota fandom is a huge part of that. I know that I can identify with her; if I was anywhere near as outgoing as she is, I'd probably act as affectionate with idols, too.
I have no idea if this is true or not or what Sasshi's fan demographics are, but I'd imagine she'd be a great figure for Japanese female idol fans, as a way of going "see, Sasshi's into idols, it's not super weird for me to love them as well." Maybe the widespread appeal has been doing good things for her popularity?
Like I said, I really don't know, but it'd be interesting to find out. But yeah, Sasshi's very fun. She even has her own term for when she gets overly enthusiastic about her fellow idols, Sasshiharassment.
Which often is directed at my oshimen, Oota Aika. So yeah, I can totally relate to Sasshi.