Rose loves music.

It's one of the very first things that River learns about her. The fifth thing, in fact, in what was swiftly becoming a long list of things to remember.

She didn't know why she was so fascinated by Rose Tyler. She knows she ought to be paying more attention to the Doctor- he's her target, he's the reason she exists, the reason for all the pain and all the lessons and so much darkness in the universe. She ought to have eyes for him, and him alone- latched onto his every word, every move, every smile and frown and hunch of his shoulders and spring in his step. Ought to have been watching this dark lord, this trickster god, as though nothing else mattered in the universe besides him- memorizing all that information and learning how to use it against him.

But it was just something about Rose. In all the stories River had ever been told as a little girl, about the enemy she would one day grow up to face, he had always just been terrifying and cruel and empty- as hollow and soulless as any ghost or shell. She had heard the stories- about him, about where he came from, about his world- about how there had been a war, and he'd lost it all, and burned his own planet and all his own people until he was the only one left, just so he could win it. She'd always thought that it would be a mercy to kill him, to carve his hearts out of his chest, because in her mind she always imagined him smiling up at her, that most intimate moment before his death, because she had been the enemy he couldn't defeat- the opponent he couldn't best- and he would know that the shell of the man he used to be was allowed to move on, because she would remain behind to protect all the things he had once tried to keep safe. Because only the person that finally killed him- bested him- would be worthy to pick up where he had left off.

This dream of hers had been rather brutally shattered the day they breached the inner sanctum of Stormcage and she met the Doctor for the first time.

He was mad. Utterly, wildly mad. But he wasn't anywhere near as empty or hollow as she'd thought. Someone had gotten to him before she had. A girl had worked her way into his life- into his hearts- before River had gotten the chance to carve them out of him to prove that she was the most important thing in his whole world. The jealousy was indescribable.

This little slip of a girl- pale, fragile, all blond ringlets of hair and painted lips and frilly dresses with lace up the sides. It had been almost sickening. River had wanted to kill her right then and there, on first sight.

She was glad, now, that she hadn't tried. Now that she knew a little more about what she would have been facing, the prospect made her feel far more sick than being upstaged by a pretty little waif had ever done.

So while she ought to have been watching the Doctor, and memorizing his every move, and using this opportunity to really learn more about him, she found herself straying to the strange girl Rose Tyler, the girl who didn't exist, who could be as gentle as an angel and as vicious as a demon all at once. This girl who would caress her cheek like a mother when she felt particularly lost or sit her down to comb her frizzy hair when it was being bothersome in the mornings or make her a cup of tea after they'd finished visiting some planet and finished breaking something.

The first thing she'd learned about Rose Tyler was that she could be more than a trifle disconcerting.

To be fair, that could probably be said of any of the three of them. But there's something especially disconcerting about Rose. The Doctor might quirk that little half-smile at the corners of his lips on occasion when he's looking at a person- the one that River had begun to read as 'You're an interesting one- my, you look tasty'- and she knew that her own habit of threatening people when she got frustrated wasn't exactly endearing, but there was just something about Rose. She could be standing there, as pretty and sweet as a little princess- and a split second later it was like something feral would raise its head inside of her and she would bare her teeth and snarl at someone.

River had asked the Doctor- just once, very quietly, in one of the rare moments that Rose was apart from his presence- what she was. Whether she was some sort of alien in Human form, or if there was a piece of her DNA that triggered some kind of agressive reaction under certain circumstances.

He had told her, eyes gleaming, that Rose was unique in all the universe. His perfect counterpart. Just as he had no people, no home, just as he was the last of his kind- she was the very first of hers. She had no people, no home to return to, because there was literally nowhere out there in the whole wide vast universe that could hope to tie her down. In that respect, the two of them were perfect for one another.

It hadn't escaped her notice that he hadn't answered her question.

All the same, while being disconcerting- as though Rose Tyler wasn't really a girl- but more like something wild and terrible convincing itself of being a girl- other than a few slips in which River had done something playful to try and break the tension and wound up triggering what she could only describe as Rose's territoriality, it hadn't really been that big of an issue. And once you knew what not to do to set her off, she was remarkably easy to get along with.

The second thing River learned about Rose was that she loved the Doctor.

Unconditionally.

It had never dawned on River that the bad guys were allowed to love. All the villainous partnerships she had ever seen or heard of before had always ended in either betrayal, death, or tears. That was how it was supposed to work- the Doctor was supposed to lure some poor soul away, trick them, make them into his weapon, and then set them loose upon the universe when he was done with them. But she had walked into the control room at night when she couldn't sleep and the TARDIS felt like it was singing in her head, and she'd walk in to find the two of them curled up together- eyes closed, though she very much doubted either of them were sleeping, and would have been very surprised if they hadn't been aware of her presence watching them. All she knew was, standing there watching this frightening little waif of a pale little girl practically cuddling with one of the most terrible forces in the 'verse, that it looked... warm. It looked warm, and safe, and something inside of her lurched at the sight of them looking so happy.

She'd wondered at first if Rose was just some Human girl he'd picked up out of the blue. If maybe she'd wind up like the others had in the stories some day- a weapon to be thrust out into the black to see what damage it could do.

Until the Crillaighek invaded Stovor, an entire species all piled into whatever ships they could get their hands on, seeking a new home world after their last one had been overwhelmed by plague. The Stova were little better than heathens. Pitiful little creatures. Anyone would have taken the obvious choice- to allow the Crillaighek to invade and settle and damn the Stova to whatever their fate may have been.

The Doctor insisted on intervening.

And the Crillaighek had taken him hostage.

This was when River had learned thing number three about Rose Tyler.

That the vengeance of a goddess spares no room for mercy.

River hadn't asked what had happened. She hadn't wanted to know. She was a Human with Time Lord DNA tacked on- just enough to be able to feel the flow of time, to sense its changing currents and eddies. Always she had seen it as a trickling stream- it had its twists and turns and ripples along the way, but for the most part, it always flowed at the same speed.

For the first time in her life, she cowered. She curled up in a corner and hid in fear because she'd felt that swelling bubble of time rise up and burst- and with it, the hurricane gale and the churning waves so swift and devastating that nothing in its wake could hope to survive.

She didn't know what had happened. It had all been too fast.

All she knew was, one second the Crillaighek high commander had been ordering Rose to surrender the TARDIS over the viewscreen with the Doctor on his knees behind him, spears pointed at his throat, and the next thing she knew she had been on the floor, shivering, with the Doctor staring down at her with that little smile, speaking in soothing tones and talking about simple things, and Rose had been leaning against him like nothing at all had happened, and the Crillaighek had been gone.

Just gone.

Just. Utterly. Gone.

Not a single ship had been left. No rubble. No ash. Not even a single trace of a ship. They might as well have been the byproducts of her overactive imagination, if not for the fact that the Doctor had that thin line of too-light blood sliced across the side of his neck, and that little smirk of adrenaline in his veins. Or whatever his people's equivalent of adrenaline was.

She'd thought it was the Doctor who'd done it, at first- until he had made sure she was alright (the way he tended to her still baffled River, sometimes. He knew that she had been born to kill him, knew she was his enemy, knew she would never stop waiting for the perfect moment to strike- and yet despite all of that, whenever they were tired at the end of the day, he would sling an arm around her shoulders and laugh. Just laugh, and smile at her, and she felt for just an instant like the Time Lord in her DNA was calling out, reaching out to him, like a child crying in the dark. It was not a feeling that her ego was entirely ready to reconcile with. Not yet, anyhow.) and had immediately gone over to Rose, pulled her into one of the most incredible, passionate kisses she had ever seen- her heart clenched a little at the sight of it, she'd long ago come to terms with the idea that there was no space for love inside her heart, only seduction to get what she wanted- and upon pulling back, had thanked her.

That was when she learned thing number four about Rose Tyler.

She was time made flesh.

Everything had fallen into place. What the glimpses of terrifying yellow eyes which swiftly faded back to blue beneath that guise of the fragile little girl truly were. Why she could be so gentle and kind to River and the Doctor- those with the blood of time running through their veins, who swam through it freely, protected and soothed by its gentle waters. And what terrible horrors of withering and decay she could inflict upon those who sought to threaten that which she considered hers. Her Doctor. Her TARDIS.

And apparently, to River's great shock, Her River as well.

She hadn't known how to treat Rose after learning that. It was hard to just sit down and let Rose tend to her hair in the sleepiness of mid-awakening as she tried to shovel some kind of food into her mouth- not when she knew that Rose could destroy her with a touch and wouldn't even need that much. For the first time in her life, she felt utterly powerless. It was not a nice feeling.

How was she supposed to kill the Doctor like this? With Rose at his side, how was she meant to end him when anything she did could be undone by a being that was time itself? When she could kill him a thousand different ways, but in the end, Rose could always travel back to before any of it happened and he would be alive again?

River had never known hopelessness until that day.

Wandering through the halls of the vast ship, feeling more and more as though they were the labyrinthine walls that would trap her forever- the place that would one day be her tomb- River couldn't help but close her eyes as the shudder traveled down her spine. She had only ever existed for one purpose. She existed in order to Kill the Doctor and nothing else. Every single day of her life had just been training and tests, leading up to that day, and after the day came she killed the Doctor at last- well, she'd always presumed she would pick up his job. Until someone came and killed her, and she would be able to rest, too.

But what would become of her if she couldn't fulfill her mission?

What would become of her if she couldn't kill the Doctor?

What would any of it mean if she couldn't use it? All the hours, days, months, years of pain- what would their purpose be, if they were never worth it because they meant she had pushed herself hard enough to defeat the Doctor?

She had no idea.

Her dream, her goal, her mission, her reason for being had been torn away before her very eyes. How was she supposed to kill the Doctor when time itself was against her? When the golden fire that ran in her veins was her own enemy?

What was her purpose, if she didn't at least have that much to look forward to?

And as she was walking, and thinking, and falling further and further into her own black mood, she had walked through a doorway, the faint sound of a few notes teasing her ears, and the next second she was staring right at the source of her problems. Rose Tyler.

And she learned the fifth thing she'd never known about her.

Rose- Time itself- loved music.

Because even as River stared at her from the edge of the wide open, circular room, the goddess in the shell of a girl spun and leapt and danced, her hair and her skirts flowing behind her like a paintbrush leaving behind a trail of ink behind itself, as she opened her arms wide and just danced.

It was mesmerizing. Colors River had never seen before sparked and shimmered in that blond hair, brought to life by the caress of light in the room. Rose looked... so happy. As though she was right here, living in this one moment, as though she had been swallowed up by the music and she was just letting it carry her body through its natural rhythm. The music was wild and dramatic and it sounded like whoever was playing it had poured their whole life into the song. She couldn't understand the words, but she didn't have to- even as her mesmerized eyes watched, Rose drew herself up at the crescendos in the song, like she was flying so high that even Icarus would have been frozen at the sight, and at the calms between the action, she would spin lazily, sadly, her dress making sorry little circles on the floor behind her like a bird whose wing had been injured and was falling.

River had seen a great many things in her time. Many of them with the Doctor himself.

But never had her breath been taken away like this before.

The song had ended all too soon, and Rose had fallen still, staring at her in the doorway with a smile on her lips. She'd known all along that River was there. That she was watching.

And then, she'd lifted up a hand towards the door, towards River- and her heart had caught in her throat, and her feet were carrying her forwards- and the two of them were holding hands, and a second too late River realized that all she could possibly do was make a fool of herself, since all she'd ever learned were traditional dances to seduce and distract, and she didn't know how to just open up and let the music flow through her like how Rose had been doing, and the next song had come on, and from the very first note, her feet had been off the ground and she had been flying.

It didn't matter that she didn't know how.

There wasn't anything to learn.

This wasn't anything like any sort of dance she'd ever learned before, complete with memorizing steps and remembering when to do what and keeping in time with the rhythm. All this dance consisted of was instinct and desire and feeling- passion drove her body to move so naturally that it meshed immediately with Rose's. And all she knew was the twist of hands and the patter of feet and the friction of two bodies together as one, guided by the masterful light of song.

She didn't know when the music had stopped.

She didn't know when she'd leaned forward.

But the music had come to an end, and carried along by the driving force of actually knowing passion without constraint for the first time in her life, she'd done what she had always been taught to do after a dance was over. She'd leaned forward and caught up Rose's lips with her own and frozen up because Rose was the Doctor's, she reminded herself, and this probably fell under the category of bad-

And then she was on the floor, on her back, in the same position she had been in the last time Rose had pulled some kind of wild and incredible stunt, and she had forgotten how to breathe because her blood was burning in her veins and her mind felt like it had been shaken clear out of her head with the ferocity of it and Rose was kissing her like she was air and water and light and everything that she had ever needed in all of space and time.

The Doctor had mentioned, once- just once, to a nosy guy on a little backwater planet who didn't like how odd the lanky, energetic stranger was and thought that his pretty blond deserved so much more- that Rose made him feel like he was the center of the universe. At the time, she'd thought it was sort of sweet and a little bit corny.

She'd never thought he meant it literally.

If this was what kissing her felt like, River never wanted to stop.

But all too soon, it ended, and she was laying there- disheveled, her hair splayed out around her- as she stared up at Rose without enough breath left within her for even awe. And Rose had opened her eyes- they ought to have been blue, those golden flames- and stared down at her with a delighted smile as though Rose were a small child who had been told her favorite toy had been fixed.

And River's mind finally caught up to her mouth just a split second after it had opened.

"Can I?"

And Rose had leaned down, and planted just the tiniest of kisses on the tip of her lips, and even as River wanted to strain forward and pull her into another desperate kiss of a drowning woman, she pulled back and stroked her fingers through River's frizzy hair.

"Finally." she whispered, and River didn't even bother trying to keep up with the tangle of arms and the press of lips and the way their hair wove through one-another's, and the way that Rose laced their fingers together and didn't let go, just holding them together like River might fall away into the black if she didn't. The music had started again, and the two of them danced together once more, and River had let go, and let it flow through her- carried along by the beat and Rose's attention until she could barely tell where one of them began and the other one ended.

And as they lay together, the over-warmth of their bodies leaking out into the light chill of the room, their only comfort the heat they were stealing from one-another's flesh, River knew that somehow, this was what had been meant for her all along. That when that word, finally, had escaped her lips- that she truly had been waiting all this time, waiting for River to catch up.

That was when she realized it. If she couldn't kill the Doctor, she'd thought that meant she didn't have a purpose. But she had to have one, even if it was one she didn't know, that she couldn't see. Because Rose was time, and she could see it- whatever it was. And if she and the Doctor had stolen River away and kept her, then it was because she was meant to be there, with the two of them, for better or for worse.

The thought was oddly comforting enough that she closed her eyes and allowed her exhaustion to take over. She could already hear Rose's shallow breaths somewhere beside her navel. For a moment, she realized that they probably ought to get dressed, but her final thought as she was lulled off to sleep was that it served the Doctor right if he walked in on them like this.

After all.

She had been jealous of Rose for so long, it was only fair to return the favor.