Enter her room once she's in the bathtub

Evellyn and the maid enter the mansion, and you decide to wait outside for a little while, while the servant heats the necessary amount of water to fill the bathtub. Finally you figure enough time has passed and you enter the mansion, secretly zipping through the rooms and hallways. It's a very opulent building, filled with expensive furniture, drapes and carpets, and old paintings telling you of the knight's ancestry. Based on these, you seem to be correct in your assumption that she has both human and elven ancestors.

You avoid being seen by any servant as you explore the building, until you find the room you've been looking for; the lady's chambers. The door is closed, so you wait until the servant girl returns with the final buckets of water to fill the tub, so that you can slip into the room without the nouse of having to open the door yourself. You then hide behind a dressing screen, set on one end of the chamber, where the lady probably normally does her dressing and undressing, shielded from the rest of the room for the sake of a sense of modesty.

You peek through the crack between the sections of the screen. The knight, a white towel wrapped around her chest, is waiting for the servant to finish filling up her bath. A high tub has been set up near the center of the chamber, a few yards in front of the large bed dominating the room. The tub is made of wood, held together by a band of iron circling it, and appears to be approximately a meter tall, allowing the bather to fully submerge themselves in it if they wanted. Steam is slowly drifting up, filling the room.

Lady Evellyn dismisses the servant, "Okay Polly that's enough. You can go now." Then she uncorks a small flask, pouring some kind of substance into the water, which quickly starts to seethe with a thick layer of bubbles.

Once the maid leaves the room, Evellyn unwraps her towel and drapes it over a nearby chair, then steps into the tub and sinks down into the bubbles until only her head remains above water.