Karl Engstrom is employed caretaker at Chris MacNeil's residence. During the party, Karl engages in a verbal brawl with a drunk Burke Dennings for terming Karl as a German. Karl also mentioned that he was in fact a Swiss. One of the notable scenes include Chris's curiosity about the presence of the Crucifix in Regan's room and questions Karl with respect to the object, while Karl nods in denial. Karl also accompanied the MacNeil family during the period of exorcism and isn't seen again post the events of the movie.
It's hinted at one point in the movie that Karl is well aware that Regan is possessed. When Karras visits the MacNeil residence, to view and interview Regan, Karl mentions to Karras and Chris "It wants no straps." The "It" he's referring to being Pazuzu rather than referring to Regan herself.