Joe Christie was a cop that worked with Monk around the time Trudy was killed. He was with Monk when he got the call about her death. He was kicked out of the police force because he was accused of stealing evidence. He is in Season three "Mr Monk and the employee of the month".
He was then working at a supermarket and there was a murder. Monk worked undercover to solve the case and found out that Joe didn't actually steal the evidence and he was reinstated. Heres a picture of him:

Hope this answers your question!!