The first ending song of K-On!! season 2 (Listen!!) shows the girls from the band meeting and befriending another girl inside of a cake. Who is the other girl?
She looks similar to Mio (Black hair, similar eyes and facial structure) and she seems to hand off her hair bow to Mio for the band playing shots. She has fairy wings, but it's unclear if she's an actual fairy or cosplaying or what.
Here's the ending animation on YouTube: https://www.youtube.com/watch?v=4EddyigHecQ