I tagged kouhai as the woman sitting in the back with her hairstyle, makes no sense to compose them like that with that level of detail and slight glow otherwise.
It should've been obvious I was not referring to the foreground girl as kouhai given that I linked to her in the comments.
I think someone ought to tweet Himura-sensei directly and ask him what name he thinks is appropiate to her. Pretty much all of the Tawawa characters' names came from him, IIRC.