Kusunoki Yua

A third-year student at Suimei Academy.
Takumi initially believes her to be a stalker but quickly befriends her after discovering that she's a fellow otaku.

Chaos;Head Noah Spoilers:
In Chaos;Head Noah, it is revealed that she's in fact Mia and the one that really died was Yua. Afterwards, Mia started suffering from real DID, believing that she was Yua while unconsciously blaming herself as the one that should have died instead of her twin sister.