While the shape of Birea's body is humanoid, including the form of her skull, the fine lines running across any exposed skin give the impression of...scales? There is a sheen to her skin as well, which adds to that hypothesis. However, it is her reptilian eyes which dismiss any notion that Birea is a mere human.
"It is an honor to serve you, Miss," Birea curtsies lightly and proceeds to help you remove your clothes as you try to sort your befuddled thoughts without staring at the maid. You are so gobsmacked that you simply acquiesce as she ushers you into an attached bathroom for a quick bath and rubdown before returning you to the dressing room. While there is a slick coolness to her skin wherever she touches you, Birea moves with a gentle and skilled grace that lets you know she is no amateur at waiting on nobility. Finally, you can contain your curiosity no longer.
"Miss Birea, please forgive me for being so forward, but...how are you..." you trail off as you still cannot think of a way to question her diplomatically and courteously.
"You can call me Birea, Miss, I'm just a maid," Birea says evenly, and goes on to answer your unasked question with a considerate smile. "And since I am human in form, I can work in the mansion without arousing too much attention. We only work in His Lordship's wing, however. Only those who are physically indistinguishable from humans can work elsewhere," she concludes, her adept hands never ceasing to tie, adjust, and pin your new outfit into position while she speaks.
Your mind is thrown into a new series of whirlpools at Birea's words. "We," you think to yourself. That means there are more people like you and Birea living and working here? Granted, you had seen the centaurs, but they had been used to pull a carriage, just as any other noble might exploit them as beasts of burden. Having changed people in high-responsibility and trusted positions within the household staff, however, was something you could not conceive of, let alone having heard of.
While in a certain sense this should reassure you, since these people did not appear to be enslaved or abused, it also brought up some new concerns. If changed people were placed in high-profile positions, it might bring unwanted attention, especially to the marquis. Why was he taking such a risk, given his situation? And would that put your family at risk, too?
As you contemplate these issues, Birea completes her duties, and you are dazed by the unrecognizable person you see in the mirror. It is true that fine feathers make fine birds, but just the fancy dress alone cannot make you such a stranger to yourself. At this point, you could truly believe the person you are seeing is of noble lineage, although you know your origins are more base than the dirt outside. You are in such a daze, you absently follow Birea down the hall and stairs to a set of doors. It is only when they open that you realize you are about to enter the dining room.