S1·E1 Destiny, the Chosen Ones

Natsuru Senō wakes up to discover one morning that he has transformed into a woman, upon which a stuffed animal tiger, Disemboweled Tiger, who introduces itself as a messenger, tells him that he has become a Kämpfer, which must do battle with other Kämpfer; since Kämpfer must be female, Natsuru transforms into a female version of himself.