Hands. Remus Lupin has always felt you could learn about a person by looking at their hands. His are covered in scars, betokening his lycanthropy, and usually dusty. He thinks that perhaps this comes from reading all of the old books in the Grimmauld Place library. Sirius' hands are calloused and rough, everyone in the house knows why. Molly's always have something she's cooking lodged under her fingernails, but the woman never seems to notice; this is something he finds endearing and incredibly motherly. Yet it's her hands that interest him the most.
Nymphadora's hands are always the same. No matter how she morphs, smaller, taller, the hands are still the same size, same shape. Remus knows this, because he has spent time studying them when she's asleep; something she doesn't know. They don't seem to fit her personality, so small and dainty; the only attribute announcing Tonks is the bitten fingernails. She tried to mask them once, charming them to grow, but they looked silly, so she quit. Remus is glad; he finds the fact that she has that habit rather charming.
Sometimes they have rings on every finger. Sometimes they are covered in dust from tidying up her latest clumsy mess, or in dirt from a squabble with Death Eaters. Sometimes they even change hue when she is in disguise, but he would recognize them anywhere. He likes the way the rest of her is so clumsy, but when she does something with her fingers, play the piano, turn the pages of a book, twirl in his hair, there is nothing awkward or inept about them.
When they sleep, he watches her for hours on end; afraid that when he wakes, she might have disappeared, that this would perhaps be just a joke. How could she really love him? He studies her then; keeping his mental image of her up to date. It was that impression in his mind that would keep him from slowly going insane when he was out there, working for the Order. He will look at her small hands and smile until there were wrinkles of joy beside his eyes; making him seem so much younger, but this is the only time he ever shows it.
Chipped, multicoloured nail polish usually coats her fingernails, which only seems to emphasize in his mind the fact that she is so much younger than he; Nymphadora can tell when these thoughts are crossing his mind, for as soon as his eyes cloud over with grief, his brow creases with thought, and his lips form a frown, her small hand will creep into his and give it a reassuring squeeze.
And he will realize again how perfectly their fingers fit together.