It's Uchida Aya, I'm pretty sure. I checked comments on Twitter to see if I could find a hint. Nothing super obvious, although one person said ・８・ which is the Kotori bird thing. But a few people noted that the artist had worked really fast, so I looked for pictures around the same time and found this of Uchida Aya wearing what looks like the clothes in the picture. Also makes sense it'd be her, this artist seems pretty into Love Live. Looks like this particular event wasn't related to Love Live, though, so I'm not adding that tag.
