Here he is .When Wilson left the hospital in episode one season 5 , Greg hired Lucas to find out plus about his own team but also to see who of them was in contact with Wilson , he wanted to know if Wilson was missing him ou asking the other team members about him etc etc .he was stunned to find that out that Cuddy , Cameron, Wilson and even Foreman had been in touch and his name (House's) had not been mentioned in their conversations .
He is a private investigator House hired at the beginning of season 5 to spy on Wilson and his team. His first appearance was on the episode "Not Cancer" and his last one on "Lucky Thirteen". He was just gone the suivant episode without explanation. I assume House didn't need a PI anymore after Wilson came back to the hospital.
