Mallory grew up harsh. She had bible thumpers as foster parents and they abused her, day and night. She couldn't leave them, so she dealt with them. But Mallory knew she was different, she was the one they hurt the most and there has to be a reason.
In 2010 she found out why. She was hybrid. A combination of an angel, a demon, and a human. And there were many like her, considered abominations, but the parents knew they were needed. The Apocalypse was there. Those demons and angels prepared for this. Though heaven and hell were enemies, they knew it had to be done. But two, very important supernatural creatures, decided that the army of hybrids, needs a leader. All of the other children were from basic demons and basic angels. Mallory was different. Her parents were the Knight of Hell, Abaddon, and the golden boy archangel, Michael. No one would have expected this and everyone is heaven and hell saw this child, this thing that wasn't born, a threat. So when Abaddon disappeared, Mallory was only 1 month old. Abaddon took care of her, well to the best of her ability. Michael couldn't be seen with her. Demons and angels alike wanted her dead. So he put Mallory on lock-down. He hid her soul, until everyone forgot about it. Until the Apocalypse was close.
She was let go of the lock-down in 1999. She was 11 when she found of the secret. So, Mallory ran. Ran far and wide to find answers, to find other people like her. Instead, she found two hunters. Sam and Dean Winchester. They took pity, but Mallory could tell Dean couldn't stand her. That's what she planned. Mallory has a way of annoying people to death. She hid from the dozens of angels that were knocking on the Winchester's front door. Micheal, himself, visited him once. Mallory didn't hide that well that time. Hoping and praying that he would notice he didn't.
Before she caught up with the Winchesters she was terrible with her powers. But after meeting them, she got the hang of it. Mallory soon learned something else. The only thing, in the entire world that can kill her, is in both Heaven and Hell. So, Mallory vowed never to go there. It was a death sentence. And she couldn't take it.
She avoided everything, but soon she grew attached to Sam and Dean and couldn't leave them. Even after the Apocalypse was over and Michael saw her for the first time. He didn't talk to her, just looked and moved on with the battle. The last she saw of him, was Sam dragging him into the cage.
By the way, this is more of a prologue, the real story takes place in Season 8 when Abaddon comes back.