Hundred

Hundred is the only weapon that can counter the mysterious Savage life form attacking Earth. To become a Slayer who wields this Hundred, Hayato Kisaragi successfully enrolls in the marine academy city ship Little Garden. However he feels a strange yet familiar sense of incongruity towards Emil Crossford, his roommate who somehow knows him from somewhere. On top of that, as soon as he enters the school, he is challenged to a duel by the "Queen" and the school's most powerful Slayer, Claire Harvey.

Synopsis from Anime News Network