Madison was a villainous female werewolf from Supernatural episode 2.17: "Heart."
Sometime before the events of the episode, Madison was mugged and attacked by an unknown assailant, revealed to be a werewolf, who was later revealed to be Madison's next door neighbor. Though she was unaware of who or what attacked her, the result of the attack was Madison becoming a werewolf.
Though not shown in the episode, Madison killed one of her co-workers in her villainous werewolf form, which brought Sam and Dean to the case. Dean managed to track the werewolf inside the apartment building belonging to Madison's ex-boyfriend, Kurt, but was surprised to see Madison as the werewolf mauling Kurt. Dean fights her off with a silver knife to her arm and later informs Sam that Madison is the actual werewolf. Madison (who is later tied to a chair) denies the claim, all the while having no memory of what happened last night; only that she went to bed and woke up in her room. Sam recalls Madison telling him earlier that she was mugged a few years back, and that led to the conclusion that another werewolf attacked Madison that night. Dean searches for, and later kills, the other werewolf—who was revealed to be Madison's neighbor, Glenn. Meanwhile, the evil Madison transformed and tried to attack Sam, only to be barricaded in a separate room.
When Madison sees the claw marks all over the walls, she realizes that she is a werewolf, but they assume that with Glenn's death, she's now cured. When the full moon rises and passes, Madison does not change, and after Dean leaves her alone with Sam, the two made love. However, Madison turned heel once again when she transformed into a werewolf and escaped out the window. Realizing that there is no cure, Madison decides that Sam has to kill her, which he later does, while Dean is in the kitchen, shedding a tear over Sam having to kill her.