Sherlock is six when it happens and he doesn't understand.
There is someone there, someone in his head, someone always with him. He sits there and watches what he does, comments on it, laughs and smiles or cries when sherlock should be crying himself. He does all of these things Sherlock's mind and Sherlock does not understand why.
He talks to Mycroft, because even though he's a brilliant six year old, he is still six and older brothers know everything at that age. Mycroft studies him closely for a moment, studies his face, his eyes. He says that it's nothing to worry about, that children his age often experience such things and he'll grow out of it.
But Sherlock doesn't know if he wants to grow out of it. He doesn't even know what it is in the first place.
The other boy, for it is a boy, is named William. He goes by Will or Liam or Willie but Sherlock calls him William because that is his proper name.
They become friends. It is an odd affair but not a bad one.
Sherlock is twelve and William is still with him.
He has stopped mentioning him to anyone but Mycroft and even then, he mentions him only in passing, like a friend rather than a constant companion. His parents still know he is there because his parents aren't stupid people but it becomes a topic of conversation that simply is not discussed.
This suits him fine.
He's read books about people like him, people who seem to be ill, who are put in institutions but he doesn't belong there. Sure, sometimes Mycroft says he does in a fit of annoyance but he never means it and his parents only take him to see psychiatrists once.
No, he's not like those other people but he does know now, know what this is supposed to be.
He doesn't believe it, of course, but knowing is better than not.
Sherlock grows up and so does William. They learn how to balance each other, how to work together to keep the body functioning and how to keep their minds from getting too bored.
William doesn't get bored easily, he likes simple things. He likes reading boring books and drawing and watching telly.
Sherlock does not.
Sherlock gets bored. Sherlock experiments, sherlock does drugs because he thinks the effects will be interesting.
William makes sure they don't overdose. William makes sure they eat. William takes them to the hospital wen things start going wrong.
Sherlock and William have a system and it is a good one. It is one that works for them. What he learned when he was young, about people supposedly like him, he never believed.
He is proof that there can be a healthy system, a system that works. he has complete faith in that and will challenge anyone who dares to try and tell him otherwise.