The problem with being a full Metamorphmagus, Nymphadora Tonks had realized at the age of fourteen, was that eventually you forgot what your true appearance was.
From the moment she was born – and potentially even before that – Tonks had almost constantly changed her appearance. At first it was just her hair color – and that continued to be the most common change – but soon it was other features as well. By the time she reached her fourteenth birthday, she had no idea what her true form was. She had at some point changed herself to look very much like her mother, but Tonks had no idea whether she actually looked like Andromeda. She kept her figure on the slim side, but since she could change her form to voluptuous with a thought she didn't know whether she truly looked like that. At least it attracted less attention.
As the years passed, she learned that most boys were only interested in her for one thing – her ability to look like any woman they wanted. Even though her hair changed color several times a day, not one boy ever asked her what her true hair color was. They were far too busy trying to persuade her to change her breasts. Eventually she took to hexing any male that approached her with such intentions, developing a rather confrontational and rebellious attitude due to that and the many detentions she received.
When she finished Hogwarts, Tonks immediately applied for Auror Academy, and breezed through the academy in record time due to her superior hexing skills (all that practice had paid off) and her Metamorphmagus abilities. Her instructors despaired at her clumsiness, as she did, but she had always been that way. Sometimes she wondered whether it was because she would often involuntarily change height from one day to the next – millimeter changes, but perhaps that was enough to affect her sense of balance. At any rate, she would never be one for stealth operations.
Then, fresh out of the academy, she met a man. Remus Lupin.
She didn't know what it was about him that attracted her, but she felt drawn to him in a way she had never felt before. He was almost painfully shy, but Tonks kept at him, trying to befriend him and hoping for something more. Remus kept playing the werewolf card to get her to back off, but she didn't understand why it was such a big deal, especially since she could tell he was interested in her as well.
And then, one day, he agreed to give it a try. It was still slow going after that, but he started opening up more to her, letting her into his life, and in turn she let him see the real her.
Well, as real as she knew.
It was after the first time they had slept together, when she awoke in his arms to find him watching her, that he asked her whether the form she assumed when she was sleeping was her true form. She blinked up at him in confusion, then asked hesitantly what she had looked like. Her nose had been longer, he replied, and her cheekbones higher. Her hair black, and her skin slightly paler than usual. After a moment's thought he added that she had seemed slightly taller, and – he blushed then – a bit more… curvaceous.
When she simply continued to stare at him in bewilderment, he gave her a kiss on the nose and told her to wait just a moment. Slipping out of bed, he left her for but a minute, returning with a pensieve. The Black family pensieve, he told her as he set it down on the bedside table. Placing a memory into it, he held out his hand to her. It took her a moment to respond, but before she knew it she was in Remus' memory, holding his hand as she looked down on the two of them in bed.
In the memory Remus was awake and looking at her sleeping form with a tender gaze that she had never seen directed her way before. It took her breath away, to see how much he cared for her.
And then, before her eyes, she saw her sleeping form change. It was subtle, for the most part – but for the hair, as it had been buttercup yellow when she had first fallen asleep – but once her transformation was complete, the woman in the bed looked quite different.
As Remus had said, the nose was longer and her cheekbones higher, her skin lighter and her body both more curvaceous and longer, and her hair raven black. She looked… rather more like her aunt Bellatrix than her mother. She wasn't sure how she felt about that.
But then she caught the look on Remus' face. The man lying in bed with her sleeping self had a look of purest love on his face, and as she watched he reached out and gently moved some of her hair out of her face. And it was then that she realized, with the force of a bludger to the stomach, that this man didn't care at all which appearance she assumed, whether it be that of a supermodel, that of Tonks with the heart-shaped face, or the regal appearance of Nymphadora. And in that moment she decided that as long as this man was by her side, as long as he looked at her that way, she wouldn't care one whit about her appearance. Whether Nymphadora, Tonks, or Metamorphmagus, she was perfect as she was.
That didn't mean that she didn't occasionally change her body to please her man, however. It was too much fun to change her breasts just as Remus was playing with them, after all.
Though he probably would never forgive her for changing her face to look like Snape's during sex.