She doesn’t know who first called her Seven. She knows that’s who she is. They all have the same name because they think of themselves as the same person, even though they know that they’re different.
She exists everywhere, but there are only seven of them, so she only exists in seven places and times at once. The seven were certain that there were no more than them.
When we say everywhere, we refer to every dimension, and every time and place. Only one of the seven are ever there. Only one person there can see and communicate with her when she visits someplace. It may not be the same person if and when she returns to a place.
She has been the same age during all of her existence. She has no memory of a beginning, and she wonders if she has an end. The seven of them have their own minds and memories, and they can talk to one another, regardless of where they are. None of them have ever died, that any of them ever knew, and she really doesn’t know how she looks. She’s never seen her reflection.
Other than those things, she’s just like everyone else, except she’s happier.