(WARNING!!! Spoilers Follow)
Ruby, portrayed by Katie Cassidy and Genevieve Cortese (Padalecki), is a demon who helps Sam in defeating Lilith, training him and convincing him to use his powers given to him by Azazel... After losing Dean, he makes it his mission to find and defeat Lilith, trusting blindly where he shouldn't.
Ruby proves herself by giving the Winchesters a demon killing blade, and by saving them several times. However, Dean still does not trust her and orders Sam to stop going down the path she's leading him on... Ruby does have ulterior motives, but Sam finds this out too late...