River Song.
The first time I met River was The Time of Angels because I watched 1-4 in less order than the Doctor and River meet. And here's Amy calling her the Doctor's future WIFE. But from the very start I loved her relationship with the Doctor. I had no idea what was going on, really, but I loved it. And that's important because I'm the sort of person who should have reacted like "HE CAN'T HAVE A WIFE" and not "OMG I NEED TO BACK AND WATCH ANY EPS SHE'S IN" but latter is what actually happened. Because she's just that good.
And at first, that's what I liked about her. I liked her relationship with him. The idea that she could wrong foot him and even better: that he'd have a nice serious relationship. When it comes to shipping and the Doctor my policy is "Yes. All of it." because he needs all the love he can get so I am never hostile to any one who loves the Doctor. And then it developed. Her relationship with the Doctor is hopelessly out of order. We don't yet know all of the implications. She was created to be a weapon against him, but, somehow it works out that she's actually his ally, friend, confidant, and wife/lover--at least at some point. But they both have massive secrets they have to keep from each other as they watch the other move, albeit not linearly, in the opposite direction. Every time, both their hearts are breaking (or possible all four of their hearts...how many does River have?). And only she can just zap in and lecture him on his naughty ways.
But when I got to know River, for herself, I loved her even more. Some people say she's a Mary Sue, some people say she is NOT, I say I DON'T CARE I am too distracted by how much I enjoy her to argue.
She's funny and clever, and somehow, out all of what happens with her, she turns out wonderful.
Rory: So...what kind of Doctor are you?
River: Archeology. Love a tomb.
Dalek: Records indicate you will show mercy, you are an associate of the Doctor's.
River: I'm River Song, check your records again.
Dalek: Mercy.
River: Say it again.
Dalek. Mercy.
River: One more time.
Dalek: Mercyyyyyyy.
River is brave and smart and caring and strong and complicated. And mysterious, still.
"The day is coming when I'll look into that man's eyes, my Doctor, and he won't have the faintest idea who I am. And I think it's going to kill me."
River: "Doctor, please tell you know who I am."
Doctor: "Who are you?"
Doctor: Time can be rewritten.
River: Not those times. Not one line. Don't you dare. It's ok, it's ok. It's not over for you, you'll see me again. You've got all of that to come. You and me, time and space. You watch us run.
"When you run with the Doctor, it feels like it will never end. But however hard you try, you can't run forever."
She has an incredible burden too. Sometimes when the Doctor takes a companion back in time, they may meet some one like Vincent Van Gogh and know they are going to die. And this can be hard, Donna couldn't bare Pompeii, she had to try. But with River, that's her whole life. Every one has things they wish they could change, and River will be no different. She'll know she can't and that to meddle might only make everything worse, but the look on her face when Rory turns up to ask for her help in A Good Man Goes to War...what she must wish she could have said to him. But it goes beyond that. She may not be a Time Lord, but she is kinda-sorta close there and with that comes curses of its own. And we don't really understand what it all means yet.
She may have an evil streak, or at least a "well I was RAISED TO KILL YOU" streak, but we know that in the end, even though she kills a good man, she turns out good and someone the Doctor will trust completely.
And it will be fascinating to see her interact with her parents now that they know.
I think she's an awesome BAMF and her story is so interesting I'm always left wanting more. And asking more questions...