The fic was Gundam Wing but with elements from Guilty Crown like the pulling weapon out of Relena (instead of Inori). It takes place at a school, and Relena was like a doll or like a toy to WuFei (I think) who is the master. He mistreats her, and in comes Heero to save her. The master changes when he is defeated by the opponent, and the opponent becomes the new master, which is how Heero becomes her master. Duo, Quatre and Trowa all have a role in the Student Council or something like it.