He was portrayed by Peter Weller, who also played Alexander Marcus.
His existence was unknown until Mary proposed that Yang had a partner during the events of "An Evening With Mr. Yang". He begins murdering people, starting with Shawn, Gus, and Mary's waitress, who served them pie. He arranges the body (with two rocks) into a Yin/Yang symbol. <p class="MsoNormal">When Yin murders Mary, just as "Mother " kills Detective Arbogast in Alfred Hitchcock's Psycho, Yin flees and then "casts" Shawn, Gus, Lassiter, Juliet, and Henry as archetypal characters from Hitchcock's canon. Supporting Mary's claim of changing the rules since Yin changed from following killing in the theme of Psycho to setting the team up as a distraction. <p class="MsoNormal">While the team is distracted by the roles they are cast, Yin kidnaps Juliet and ties her to the top of a a clock tower. He then travels to the airport, where Buzz McNab is picking up Shawn's girlfriend Abigail Lytar. Yin then drugs McNab and kidnaps Abigail, forcing Shawn to choose which one he wants to save. Yin ties Abigail to the bottom of a pier, and forces Shawn to make another decision - catch Yin or save Abigail. Shawn initially picks Juliet, due to knowing her location, but thanks to a hint from Yang, discovers where Abigail had been hidden. While he trusts Lassiter and Gus to save Juliet, Shawn and his father go to the pier to save Abigail. Shawn comes face to face with Yin (albeit masked), and upon realizing that she was under the pier with high tide coming in, he is forced to allow him to escape, once again. The episode ends with Yin stroking a picture of Yang with a young Shawn.
He makes another appearance in "Yang 3 in 2D" where he and his new apprentice, Allison Cowley, lure Shawn and Gus to his home to kill them; Allison pretends that she was kidnapped by Yin to trick Shawn to follow her. They get help from Mr. Yang to find the clues to lead them to Mr. Yin. Shawn and Gus are eventually led to his home where they are held captive and await their impending death. He admits to Shawn that at first he was only interested in Shawn due to his skill; however, after Yang sacrificed herself and ruined the plan in order to save Shawn's mother, he developed an intense hatred of him.
<p class="MsoNormal">Just before he kills them, Yang comes into the room, revealing that she is Yin's biological daughter. After convincing Yin to forgive her for betraying him, she kills him with his poison originally intended for Gus (a personal mixture or cyanide, strychnine, atropine and boat cleaner, affectionately designated "The Last Kiss"). As he dies he admits that he never loved her. Though she gleefully replies "I know!".
- It is highly possible that Mr. Yin abused his daughter, Mr. Yang. Whether physically or emotionally or both, it resulted in her becoming the infamous serial killer she became known as.
- Though Peter Weller played him for his last appearance, in his first appearance, he was portrayed by Christopher Turner.