After observing for some days, I can surely call this a fake. Her name is Miura Akane. She looks like at the same year as Azusa. She would be playing euphonium (picture above) if she joins the band. It is weird for an euphonium in a, I say generic pop band isn’t it?
In the fact, the official site of K-ON!! series has not said anything about this. But besides the controversy, I still acknowledge her cuteness. Her pinky hair reminds me to Hinagiku. Her ponytail, sporty-look, zettai ryouiki and cute eyes, they’re too nice for a joke.