This one has been climbing up the ranks of my favorites, Hina Hikawa. That's because of my increasing interest of her personality. Her lack of knowledge of how to treat people, including her teasing of Aya and being unaware of Sayo's feelings.
Hina's mildly rude and oblivious traits even up with her being a prodigy, which I like a lot, as it doesn't make her perfect. Some people would say she's too "rude" in a way, but that's why I love her.
Her singing voice and design are both really cute as well!