Karen Haas is a U.S. marshal who Jake and Holt under their aliases, Larry and Greg, meet up several times with while in Florida. She quizzes them on the backstories of their aliases and updates them about Figgis.
Throughout the Series
In Coral Palms Pt. 1, she is first seen when she met up with Jake and Holt in "Location 1", a parking lot. The three of them talk inside her car where she asked them about Greg and Larry's lives like where Greg went to college (Ohio State, major in Communications) and Larry's favorite movie (Failure to Launch). She then told them that there was an informant who told them where Jimmy "The Butcher" Figgis was and she, along with 50 other federal agents, went and raided a warehouse. Unfortunately, Figgis was never in that warehouse. She then announces a pop quiz and asks Jake who the female lead was in "Failure to Launch". He answers Kate Hudson then she corrects him, saying "Sarah Jessica Parker, man. God, it's like you wanna die."
In Coral Palms Pt. 2, she calls Jake and Holt for a special meeting in a movie theater. She starts with a simple greeting and asking them how it's going in which Jake replies by simply answering "the normal checklist stuff". She then proceeds to ask them if no one was going to ask her how she's doing. Holt said he didn't know it was a personal conversation but she shuts him down and tells him that it's official business and she was just testing him because "Greg is the type of guy who cares about his friends and can sense when something is off with them." Holt then asked her if something was off and then she breaks away from her professional tone and tells them that she met someone and it wasn't her husband and nothing's happened yet but apparently her "entire body is on fire". When asked if what she was saying was still official business, she goes back to her stern tone and announces a pop quiz. She told Jake a scenario that was implied that's similar to her current situation and asked him if Larry would do it. It was then mentioned that this person was really young. Holt then answers the scenario saying that it seemed that Larry has needs and those needs deserve to be met. Before she leaves, she says to Holt that he's really good at being undercover and that both he and Jake passed the test then it's unveiled that she brought along the person she met calling out, "Marco! ¡Vámonos!".
By the middle of the episode, to prove to the sheriff that they were telling the truth, Jake and Holt told him that they could ask Marshall Haas. Jake took the phone from the sheriff after he told them that he was talking to a guy. Figgis was on the other line and says that he has Marshall Haas.