Senou Natsuru was an average male student and young Japanese citizen. But one day he got suddenly turned from a normal human into a 'Kämpfer' and is forced into the fights between them.
He is the maincharacter of the story and the only current guy in the series, who wears a Kämpfer bracelet, that enables him to transform. But for whatever the reason all Kämpfers have to be female and so his gender switches when he needs to use his powers.