About what was discussed on previous page, I think vol.15 is very suitable volume for a movie/OVA.
For a new freshman on the upcomming volume's cover, I would assume that she's a reletive of Frenda . Anyway, no matter my guess is right or wrong, from how Hamazura look at her, she might be a new person in his harem.
As for Touma, maybe someone found him in his coma state and kept him somewhere in the world.