Tensei Shitara Warui Kuni no Musume DeshitaTensei Shitara Warui Ko deshita; 転生したら悪い国の娘でした。; Tensei Shitara Warui Kuni no Musume deshita; Tensei Shitara Warui Kuni no Ko deshita; I (♂) was Reincarnated as a Female Member of the Enemy Country; О моём перерождении в мире демонов; 轉生之后變成壞女孩

Childhood friend (♀) gets attacked by something (♀), Main character (♂) dies in crossfire. Main character (♂) is reincarnated as the living weapon (♀) of the demon lord (♂->♀) that just killed him, while his childhood friend (♀) is apparently now the hero (♀->♂?). Reincarnation + Isekai + Double(?) Genderswap.