Aaron Hayden is the younger brother and murderer of coach Lynn Hayden.
A few years prior to his episode debut in "Mr. Monk and the Big Game", he murdered a woman named Paula MacGoohan. He left DNA evidence all over the country club were the scene occurred, but the police had nothing to match it to, and thus he was never caught. However, a few years later his sister accidentally started a brush fire, apparently by accident, that didn't kill anyone, but did cause a lot of proper damage. Lynn, wanting to do the right thing, decided to turn herself in, despite Aaron not wanting her to. He knew that if she turned herself in, the police would take a DNA sample of her, which would then be a close match to the DNA (his) at the country club, being they were siblings, and he would get caught. He then murdered his sister to prevent her from turning herself in. When Adrian Monk later solved the case, he tried to run away, but was caught by Leland Stottlemeyer and Randy Disher and arrested.