When she taps on the window of his front room, he goes to her and he smiles and lets her in. His smile is genuine but her return smile is not; it is not hers, really, because she transforms like he transforms. Both are natural and involuntary. While his is triggered by the full moon, hers is triggered only by him.
Every second Tuesday of a month this tends to happen, and she can only imagine the hell he goes through while in limbo. She can imagine the lies, and recites them to herself in the mirror. "Sirius is in hiding. Sirius visits once a month. Sirius Black is not dead," she says, and manipulates her vocal cords until her voice is his, changes her hair until its length and color is his length and color, forces her eyes to reflect his eyes. She is always startled to find the emotions in them match as well.
Nymphadora does not think of what will happen if he awakes when she transforms in the night. Rather, she knows what will happen but does not allow it. She remains conscious and watches, and maybe for one minute (two minutes, three minutes, five minutes) she is herself and he is himself and it is different. She has practiced not flinching when he says his name. She does not start at the long, black, scraggly hair, but has copied her cousin's movements fluidly. She sometimes tests, the few times she grows angry at her passivity, just how much of Nymphadora Tonks can be in Sirius Black before the lover notices.
In the mornings, she leaves the same way she has come. Moody looks at her strangely on occasion but she reminds herself that, like a cut in her mouth which will not heal (because of tongue-ing), everyone thinks of the whole Sirius-Remus thing from Remus' point of view because he is the only one who hasn't changed. Sirius has died, after all. This makes the werewolf the static character. And when Remus whispers things in the dark, when he runs his hands through her-his-Sirius' hair and asks her-him-himself questions, she answers truthfully. Will he-she-they ever leave him? No. No.
She knows this, and she knows this. She will not leave him, will not watch him crumple and die like her cousin has done. The cousin calls to her from the portal, and Moody calls to her from the broom ahead, and Harry calls to her from school, and her own self calls to her from the mirror. She does not hear them, only hears the man in the patched robes, only worries when the boundaries between herself and other people have become too blurred to feel anything but the missing part of him Sirius used to fill.