Her name had not always been River Song. It was far more complicated than that.
Nor had she simply been Romanadvoratrelundar. Nor Rose Marion Tyler, nor Sarah Jane Smith, nor even Leucadia, all those years ago. Not quite. Though the last—the first time—came closest. She had thought so at the time, anyway.
She had been simple, in the beginning, at least compared to what she later became. Not so, now.
What she was, when everything came down to truth, was Time. Almost. In part. No one being could encompass Time herself, but as far as anyone could, she did. And what she would always be was there when he needed her. What she had done was love him when he needed to be loved, regardless of the circumstances or the method. Where she had always been was by his side, seen or unseen, but always there.
In the beginning, she was the Time Lord who'd loved him too much to leave him, who had died too quickly for regeneration—but had the split second to dedicate herself to something greater. To become something more. Something he would never understand, and something she could only begin to ask for. But she'd done it. She'd done it as Leucadia, as Sarah Jane, as Romana, as Rose, and of course, as River Song. Though River Song was only the latest name, the latest self. Someday, there would be another, though River Song would last longer than most.
And she as River understood far more than any form of her had throughout the centuries. (Hearing him say he was nine hundred years old made her laugh; they both knew better). She as River felt the need to tie it all together, to make sense of the chaos that had been his life, always wishing she could give him more than he would accept.
There were reasons he was never allowed to read that book.
They'd been young and brilliant together, unstoppable and inseparable. Inordinately proud of their little boy, especially when he grew up to become a man. They were unorthodox, uncanny, perfectly suited in many ways—though too much perfection would not have suited them at all. They fought like they loved, often and explosively. She was always right and he was never wrong, and they had been happy.
And then she'd died, and she'd done the impossible.
He'd tried to forget her, and she understood. After a freak accident stole from him his very reason for living, he'd contemplated dying when forgetting failed. He'd even tried, once, and only succeeded in estranging himself from his only child. Years would pass before he unburied himself from work enough to realize what he'd lost, and then there was a little girl named Susan, who ran with him and made him live again.
The Doctor thought he'd made himself forget the pain, forget her, Leucadia, so much that he hardly even remembered her name. He never spoke of it, just went on. His first incarnation aged at the gallop, eventually wearing out.
She hadn't been Susan. She hadn't needed to be, and even Time, for all the ins and outs and out of order, for all of the wibbly-wobbliness of the art, could not imbue a spirit into an already born granddaughter.
Years passed before he was ready to love, needed to be loved. This time he needed a friend, so she was there, and she was what he needed. As she would always be.
Sarah Jane Smith would have stayed with him forever, had he asked. But circumstances got in the way, and then she oh-so-foolishly decided to be a Time Lord again, thinking that this time she could follow him anywhere, and that he might even let her.
She had been right, but she had also been wrong.
What she hadn't quite realized before Romanadvoratrelundar was that he wasn't ready for that kind of love, had acquired a terror of being held down and defined by conventional standards. And then she was stuck in that iteration because she had chosen to be a Time Lord, and thus regenerated instead of dying like a more mortal being. Time Lords existed, for the most part, outside of the normal timeline, but they did share a common timeline with one another. Because of that, she'd not been able to shift to someone he needed for far too long.
She was more Romana than anything else when she'd begged him to do the impossible and end their world. What she didn't tell him was that she knew he would live, and she knew they would be together again.
Somehow, in mixing herself up with Time, the Gallifreyian woman who had been Leucadia/Sarah-Jane/Romanadvoratrelundar had gained the knowledge that the Doctor was different, and that he did indeed need something greater than himself to hold on to. He had been called Time's Champion, more than once, and she thought the title fit.
Only champions endured so much pain with so little complaint.
After the Time War, she came more frequently.
She was the Bad Wolf. She was Rose Tyler. "My Doctor," she had whispered. In the grip of so much power and such a strong need to save him, she had almost said his name, almost told him everything. She had been ready to die to save him, right up until he died to save her.
This time she thought she could stay with him forever, and in his loneliness he finally did need the same love he'd forgotten once sharing. Until circumstances again intervened, and the simple human life she'd lived out with another him who needed her just as badly was not quite enough. Wall between the universes or no, she could always feel him, and although she was never torn between her two Doctors, not a day went by that she did not bleed for him.
So the next time she tried something different. Now she understood that he could not believe that anyone could stay with him forever—her first death had broken him of that hope, and losing Rose hurt too badly for him to hope that way ever again.
Thus she became River Song, and lived her life out of order. This time she was obsessed with record keeping and with memories of the past—hence the blue book. And so the memories were written down, in a book she always carried but he knewbetter than ever read. But she had to record them. She had to remember, even though she knew that she ought not.
She was hurt as badly as he was, in her own way. Just as lonely.
But he healed her as much as she healed him. Shortly before she as River died, seeing him for the first time that he met her, she finally realized how very much she had helped him—and how very much he would come to understand even without reading that little blue book. And in those crucial few seconds, she knew how to win his trust, what to whisper in his ear.
Because she remembered his name, from marrying him so long ago. She knew him as no other did. And she was there when he needed her, even once everything changed and he whispered her name in return.