Ruby

Ruby first appears to watch Sam as Bobby and the Winchesters track the Seven Deadly Sins demons. She joins in the fight as Sam is attacked, and uses a knife that appears to be able to kill demons, and then disappears mysteriously.

Deeper into Season 3, She appears more frequently, claiming that she is able to help Dean escape his contract. When it came down to it, however, she is betrayed by the Winchester brothers, but returns to "help" them. Her fate, at the end of the season, is unknown as Lilith possessed her body.

Season 4
After Lilith possessed her original body during the end of Season 3, Ruby returns, in a different body, to help Sam figure out what pulled Dean out of hell. However, Dean is unaware that Ruby has returned until he sees Sam with Ruby in "Metamorphosis". Apparently, her motive is to help Sam increase the efficiency of his powers. However, still being a demon, Dean doesn't trust her. The knife that Ruby carries is now with the brothers.

In "I Know What You Did Last Summer", Ruby drops the Winchesters a tip about a girl Anna Milton, who is wanted by demons. This prompts Dean to ask Sam why he trusts her so much. Through flashbacks, we learn that Ruby returned after promising Lilith that she would kill Sam. She betrays Lilith and assists Sam once again. Sam also scorns her for using a living body, prompting her to possess a brain-dead coma patient. After this, she begins teaching Sam to use his powers. She seduces him at one point, and gets him off his self-destructive path. Back in the present, Ruby contacts Sam and Dean by possessing a maid, giving them an address. They head to the cabin, where Ruby has stashed Anna. After a few minutes, Anna is stashed away again just as Castiel and Uriel arrive, declaring that the girl has to die...

Trivia

 * Ruby, in season 4, is almost always referred to as a "Bitch" or a "Slut".
 * Apparently, at a time, Ruby used to be human.

Appearances

 * "The Magnificent Seven"
 * "The Kids Are Alright"
 * "Sin City"
 * "Malleus Maleficarum"
 * "Jus In Bello"
 * "No Rest for the Wicked"
 * "Lazarus Rising"
 * "Are You There God? It's Me, Dean Winchester"
 * "In The Beginning"
 * "Metamorphosis"
 * "I Know What You Did Last Summer"
 * "Heaven and Hell"
 * "Criss Angel Is a Douchebag"