Seikai Trilogy

Volume 1: Crest of the Stars (Seikai no Monshou)
Volume 2: Banner of the Stars (Seikai no Senki)
Volume 3: Banner of the Stars II (Seikai no Senki II)

After his home world was taken over by Abh - a space-faring race that rules over half of humankind - a boy named Jinto becomes a member of Abh nobility, and enrolls in their military school.
After asking a young Abh girl who was sent to pick him up for her name, he later discovers that she is one of the potential heirs for the whole Abh empire.
Will their friendship survive that discovery, and start of a war that will engulf whole galaxy?