It looks like I'm not the only one who noticed this character. In the commentary he/she calls her merely a background character (モブ, mobu, literally "mob"), but if that's true I'll eat my hat. It's very unusual for a background chatacter to have a distinctive character design, since it distracts from the mains. On top of that, she's also a head taller than everyone else She seems intended to draw attention, so by Chekov's Gun I suspect she will be important.