She loved him and he broke her.
Some say that it's impossible for someone to fall in love in a week, but that's what happened. So, thanks to them, she had to become a hunter. A solo hunter. No knowledge, no weapons, just her knowledge of what the Winchesters had taught her, her guts, and what she knew from the crap she had been able to look up on her stolen laptop.
Rock salt wards away sprits.
Angles are basically immortal, but an angel killing blade can kill them.
Some people were gifted by Azel.
The Levitations were also out to eat everyone on the planet and kill the Winchester brothers.
The apocalypse had nearly happened more than once, before she met the Winchesters, and twice for her to stop it.
She slept with Crowley to get them out of hell.
She learned that hunting is one-sixth luck, one-sixth knowledge, one-sixth weapons, and three-sixths having a partner beside you.
She had seen hell, heaven and earth. She had walked the earth during Lucifer's rule, the year that never was, she had to face death and pain and loss, but she never felt stronger.
A hunter she had entered. A warrior had emerged. She still had a respect for human life, but if one or two died, it was better than the whole world.
Right?
In all those years of hunting, she had lost her humanity. Big time. She had died, to many times to keep count, and became immortal after the first few times- personally she thinks that Crowley got tired of pulling her out of hell.
She knew the Winchesters were there somewhere, but after a few weeks of searching, she gave up- it was clear that they didn't want to be found.
Little did she know, Sam, Dean, Mary, John, Bobby, Castiel, Gaberiel, Crowley and all the others were forced to watched as she hunted. Every single little hunt, every single little fight with the demons, with the angles- even made to watch as the Devil came free of his cage and tourted her for a year, turning her half demon, half angel. She had the abilities of both, but she was as far from the light or the dark as anyone could imagine.
Crowley owned her soul, but Dean Winchester had her heart.
She had tried to kill herself, more than once, but it never worked. Even the first time, before she was immortal.
She was Melanie Golden, half angel, half demon, hunter, warrior of heaven and hell, Queen Of The Crossroads and, although she didn't know it, keeper of Dean Winchesters heart.