Lucas Douglas is a private investigator. He is hired by House in the fifth season episode "Not Cancer" to spy on House's team and gain information about them. Among other things, he finds out about a secret bank account started up by Taub's wife of which Taub is unaware. Impressed by his thorough efforts, House decides to put Lucas "on retainer".

Next, in "Adverse Events", House uses Lucas to gain information about Cuddy's personal life. Lucas, though, had also begun to take a romantic interest in Cuddy. In "Lucky Thirteen", House uses him to spy on Wilson because he suspects he was lying about where he was one morning. Lucas finds out that he's dating a hooker and that he's doing drugs. House then figures out Wilson was trying to throw him off because he knew Lucas was following him. It is also revealed that Lucas got the keys to all of his fellows' houses. House also used him to dig up info about Foreman, but could not find anything interesting.

Michael Weston reprises the role of Lucas during season six in the episode "Known Unknowns". Lucas had begun dating Cuddy. In the season 6 finale, Cuddy breaks off her engagement with him and ends their relationship so that she can be with House.