History:

Once upon a time, Ruby was a human -- sometime in the 1350s to be precise. But, Ruby was dabbling in witchcraft and she got in deep. She sold her soul to a demon named Tammi to boost her witchy talents and wound up getting sent to Hell for her efforts. It should be noted that through some of Ruby's comments made to Dean and Sam throughout season 3 and 4, particularly about remembering what it's like to lose someone and her displeasure to run into Tammi again, I generally make it a headcanon rule that she lost someone close to her (specifics of which she genuinely doesn't remember) in the process to becoming a witch, or perhaps even that was why she began to dabble in witchcraft. In any case, over time, Hell? It changes a person. It rips them up and spits them out and carves them into a new animal. Ruby's kind of the cautionary tale of what would happen if Dean had hung around in Hell too long because after hundreds of years of that torment, she turned into a demon. Her very humanity was carved out of her. The general step-by-step is that Alastair makes sure that his students carve everything out. Leave nothing that might lend to a hint of sympathy, but Ruby still has flashes -- memories. She remembers what being human, feeling things, and even being hurt feels like. Which is pretty much why she gets commissioned by Azazel to rope the Winchesters into releasing Lucifer -- because who better to toy with people's feelings than one of the few demons who can still understand them? And so, Ruby got her very own special covert operation. Only Lilith, Azazel and Ruby were privy to this information, and she set about getting the boys on her side.

She first appeared to them after the Devil's Gate was opened. She stalks Sam and Bobby on their hunt for the seven deadly sins and, when a confrontation ensues, Ruby uses her magic demon-killing knife to take two and a half out. (Sam helps her on the third.) She knows that by making herself A.) a useful presence, B.) a reliable one, and C.) mysteriously badass, she'll get not only their attention, but spark enough interest to eventually warrant some trust. After that she pretty much continues to stalk the boys, but she only makes herself known to Sam and taunts him with pieces of information about Azazel and the whole Antichrist thing. She calls herself a hunter because she knows that she needs to get him interested and eating out of the palm of her hand before she continues on to reveal who she really is. He won't trust a demon out of the gate, she needs to lead him into seeing that she has something to offer him, and then she can lay the smackdown. As she tries to make promises that she knows how to save Dean's soul, Ruby reveals herself as a demon to Sam during their second meeting in an attempt to show off how ~straight forward and honest and trustworthy~ she is. After that, Ruby goes to help Bobby fix the Colt -- despite him trying to threaten her with it -- to make herself, again, seem more trustworthy and like she's trying to genuinely help them. To be a good guy, you need to help the good guys, even when that means fixing the things that can fully and horribly murder you. When Sam threatens her with the Colt, she bribes him into not killing her with further vague promises of Dean's being released from his deal and they make a begrudging pact to help one another out.