They had been servants to the Kisaragi family for a long time. Mamoru doesn't have any spiritual powers, but he takes care of the housework, home finances, computer programs, and does management of Ayaka's cases.
Mamoru knows enough not to lose to normal adults, but he can't ever go against Ayaka. It's not because he is a servant, but more like he is a younger brother who had to cover for his older sister. If Ayaka gets a boy friend, Mamoru will be the one who cries. He's the only boy to which Ayaka's sex appeal does not work against.