He was portrayed by Christopher Turner and Peter Weller, who also played Alexander Marcus.
Mr. Yin was an infamous serial killer often taunted the police in a Zodiac type of fashion before he killed his next victim. He is arguably the most infamous, darkest and cruelest villain on the show. He abused and brainwashed his daughter, Mr. Yang, into being a psychopath. He would commit crimes while under her own alias, leading everyone to believe she was the killer herself. He would mail the police letters about how they could find the victims before he'd kill them. Although he did not play fair as he would change his own rules once his plans were discovered. Mr. Yang betrayed Yin out of her admiration for Shawn and helped save the later's mother. She was later arrested without anyone knowing about Yin. Out of anger of losing her as an apprentice, he targets Shawn in "Mr. Yin Presents..." and reveals his existence by killing a waitress who served Shawn, Gus, and psychologist Mary at a diner. He begins targeting them in ways that mirror that of Alfred Hitchcock films. Once they find him, he kills Mary in a way similar the staircase scene from Psycho, and uses his death as a distraction to kidnap Jules. Mr. Yin later kidnaps Shawn's girlfriend, Abigail, and gives him the options on who to save, trapping Jules on a clock tower and Abigail under a dock. When Shawn finds Abigail, he meets a masked Yin, but is forced to let him go and save Abigail. Jules is later saved by Lassie.Needing an apprentice, Yin makes one of Allison Cowley, and has her pretend to be a former victim of his in order to kidnap Shawn and Gus. This plan works and Allison kidnaps Shawn and Gus, tying them both to a chair in front of Yin. Yin has her leave and attempts to inject them both with poison. However Shawn and Gus manage to keep Yin distracted long enough for Yang to arrive. She pretends to want to come back as an apprentice, only to inject Yin with the poison he intended for Shawn and Gus, killing him, but not before he admits he never loved her.