Peter Breen was a construction foreman.
He had an affair with Stephanie Preston, but he later decided to kill her, having one of his construction workers, Eddie Murdoch, commit the deed and also loaning him the key set (which included a Ferrari key and his house keys), as well as burn her house to the ground. He was also indirectly responsible for Monk being temporarily blinded, as Eddie Murdoch had to kill a Fireman so he could go back to the house and retrieve the keys on the coffee table, of which Monk was blinded by him in the encounter from acid being thrown into his eyes. Afterwards, Monk, Natalie, Captain Stottlemeyer and Lt. Disher questioned him at the construction site, but was later informed by Eddie Murdoch that he has his keys just before Monk identifies Murdoch from his footsteps. After Murdoch ended up killed as a result of slipping off a high level of the construction facility, Breen posed as a cadaver and hid among the morgue, having anticipated the likelihood that Monk and Stottlemeyer will try to find the incriminating key set. He then restrains Stottlemeyer to a gourney before attacking Monk. However, he ends up shot by Monk as he recovers his eyesight in the nick of time. It is unknown whether he survived the gunshot wound. Based on his speech patterns and accent, Breen was most likely from Ireland.