I think from the look Fate gives her on pg 34, she might be the 'master' and Hayate just says 'he/him' to up the mystery. (Not to mention that if the sex of the person is unknown, 'he/him' is generally used, too. :D)
Maybe she's pretending to be a maid to win Nanoha over properly and not just because of her status as 'master', heheheh. :3