Mary Lightly (his father's name was Mary, his grandfather's name was Mary, and his great-grandfather's name was Craig) is the foremost expert on the notorious serial killer known as Mr. Yang. In fact, he had devoted most of his life to that subject. When Mr. Yang was finally captured and imprisoned, Mary wasn't sure what he'd do with his life. Shawn Spencer suggested racquetball.
About a year later, Mary made contact with Shawn and Gus, telling them that he was convinced that Mr. Yang had a partner, named Mr. Yin. Shortly later, Mr. Yin began a killing spree that placed Shawn, Gus, Juliet O'Hara, Carlton Lassiter, Henry Spencer, and even Mary himself at the center.
Thinking he could beat Mr. Yin at his own game, Mary took an opportunity to kill Mr. Yin himself, only to be killed in the process. Moments before dying, he asked Shawn if there was racquetball in heaven, and Shawn said there was.
At Mary's funeral, Shawn and Gus paid their respects by wearing racquetball uniforms and holding racquetball rackets. Only one other person attended the funeral - an older woman (she wasn't identified, but it's likely that she was his mother or some other relative).