Baldr Heart

Sou was a mercenary who piloted Simulacra—mechanized combat units—until an incident stripped him of his fighting ability and his livelihood both. He retreats to his foster father's island home of Wadatsumi, his former partner Yuuri trailing behind him uninvited. A chance rescue of an amnesiac girl named Tsukuyomi lands her in his care. When a mysterious job offer arrives to investigate whether Kouka Academy students are connected to the terrorism gripping the island, Sou goes undercover—only to be confronted immediately by student council president Mao and her Simulacrum. Then a being calling herself a fairy appears and begins merging with his own.