Is this an artist original character? I don't recall a character in GuP with dark hair and twin braids like her.
I assumed that it was a character from another series cosplaying in the Ooarai uniform. I haven't seen this particular artist use this character elsewhere, although that doesn't mean that it's not an original character. Unfortunately, there was no commentary on that tweet, and the tweet is dead so it's no longer possible to read any of the replies.
