Hoshi no Ondo, performed at the Shibuya AX concert. This performance featured Nacchan, who sang in the recorded version on the A1 CD release. The first time I watched this and saw her there, I bawled like a baby through the whole song.

“Nage kissu de uchi otose!” performance from Team A’s 3rd Stage, “Dareka no tame ni” (Uploaded by CJKpopfan @ Youtube)
Nacchan is one of the three backups of this song, so you don’t see as much of her as the others or hear her voice, but she looks darn cute when you do see her.