Diamonds and Dragons - Gary Henderson

Charles, a lonely young outcast with an ugly scar and ungainly wings for a dragon, perched on the castle wall and listened to stories. The boy inside that castle window befriended him, and the two grew together to become kings of their peoples. Book One, 'Charles, A Dragon,' and Book Two, 'The King Arrives,' tell their stories. Now, in Book Three, their kingdoms are under attack by nations far over the eastern mountains, for unknown reasons.

Their friend Charis is reunited with her parents, gone since her birth.  Together they pursue the evil that is driving the invaders, and finally confront it on the very steps of Castle Romain.