Au where Ruby has a redemption arc of sorts, but only out of love and dedication to Sam. She still sees Sam as this important chosen one who can rule over hell, but she told him the truth in time, and seeing his reaction and how much he hated it, she chose him over Lucifer.
She still hates Dean, and doesn't really care about humanity, but she's there for Sam. And well, yeah she was going to betray him, but she didn't. So Sam doesn't have the heart to kill or ditch her, shes still the Ruby he spent months with.
And so she becomes this weird third in the impala who is constantly competing with Dean for Sam's love and attention. And it's driving Sam crazy. (Especially when his older brother even goes so far that he can give him everything Ruby does, sex and all)













