She was played by Mena Suvari.
Allison Cowley is a young woman who appeared at Santa Barbara police station claiming to have escaped from notorious serial killer Mr. Yin. She led police to the house where she claimed that she was held captive in, only to discover that it was occupied by an innocent family. Still believing her claims that Yin is after her, Shawn took Allison to his house to keep her safe. However, Allison is later abducted from Shawn's old bedroom, with the yin and yang symbol left as a calling card.
While watching a videotape featuring Mary Lightly (made before his murder) discussing Yin (among other subjects), Shawn and Gus see footage of Allison held captive in what is revealed to be an attic in Yin's own home. Shawn and Gus rush to Allison's rescue, only for the "hostage" to turn villainous and reveal that she is Yin's new apprentice, and that this was an orchestrated trap set to catch the pair. Allison easily breaks free of her handcuffs and points a shotgun at Shawn and Gus, all the while boasting about her ruse as she led them to Yin.
Allison is later sent away by Yin and encountered Juliet; giving a fake sob story about her "abduction." She slips up and mentions Shawn and Gus being in the house, leading to the villainess fighting with Juliet. Allison is defeated by Juliet and is arrested, but as she is driven away, Allison gave an ominous stare at Shawn and Gus.