Mr. Yang is a notorious serial killer who likes to challenge the most apt detective around to a "game" which puts countless people in danger, including the person who Mr. Yang challenges.
In 2009, Mr. Yang emerged from a nearly 13-year exile to challenge Shawn Spencer to a game. After Shawn "won", it was revealed that Mr. Yang was actually a woman and she was watching Shawn both before and during the game that she challenged him to. In the end, she was arrested and put away into a psychiatric facility, where she then began to write her book.
Unknown to Shawn or his colleagues (in 2009), Mr. Yang had an accomplice named Mr. Yin. Yang's counterpart made himself known a year later when he kidnapped Juliet O'Hara and Abigail Lytar and put them both in fatal situations, in an attempt to make Shawn choose who to save. In the end, Shawn was able to save both of them, but in doing so, he had to let Mr. Yin escape.