I don't know if she is an nihth, but she defiantly has something to do with the cult. If you break into her room there is a chance that a portal will be in her room. I don't know the riggers to see it but IIRC if you interact with the portal it will send you to your room and will then be gone for the rest of the game.
To unlock the first girl, you need to go into the city at night and go to the convenience store. (on the map north-east of the tower). I thin you need to meet her there twice to progress her quest.
The second girl can be met after you have done the panty quest a few times. After classes end, talk to the guy with a quest for you to steal panty's for him (right of the school building). Go to the second floor 1 and break into room 7. In the room, if you have progressed far enough, you will see some panties on the stool in the middle of the room. Interact with them and you will be able to progress he plotline.