Tsubasa: Those with Wings is a sci-fi series by Natsuki Takaya, who's best known in the US as the creator of Fruits Basket. I bought my copy used and, since the series was originally published by Tokyopop and never got a license rescue after the company imploded, used and library checkouts are pretty much the only ways to get it these days.
This review includes spoilers (both the major series ones - go back and read the earlier volumes if you haven't yet - and a few that I consider more minor, a little info on how things turn out for some of the characters).
Review:
Kotobuki and Raimon are still separated, I believe due to the bomb in Raimon's head (if I remember right, it would go off if he went with Kotobuki on her trip to what was left of Japan). Kotobuki has spoken to the last living Japanese person, a neurosurgeon who tells her the true form of the Tsubasa: they were actually genetically engineered bodiless brains. Only two brains were ever successfully created, and one day those brains spontaneously became a boy and a girl, Rikuro and Kayo.
Kayo has been missing for a while, and Rikuro can't function as the Tsubasa without her, but the neurosurgeon should be able to remove the bomb from Raimon's head. Unfortunately, there's one big problem: Raimon has been captured. Colonel Hil Gil plans to use him to awaken Kayo and, with her, the power of the Tsubasa, which he plans to use to grant his own secret wish.


