She was played by Emmanuelle Vaugier.
The episode had Sam and Dean investigating the brutal killing of one of Madison's co-workers, suspecting it to be the work of a werewolf. After Madison tells the boys that she had been stalked by her ex-boyfriend, Kurt, Dean tracks him, while Sam stays with Madison. After hearing something from Kurt's apartment building, Dean crashed in to find Kurt being killed by Madison, who was revealed as the werewolf. 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, Sam had to deal with Madison, who transformed and tried to attack him, only to be barricaded in a separate room.
When Madison sees the claw marks all over the walls, she realizes that the boys were right and she actually 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, Sam woke up to see Madison transformed into a werewolf and escaping 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.