Marcus was a demon who possessed as an orderly at a mental hospital.
At some point, a demon possessed the real Marcus who worked at the Northern Indiana State Hospital, possibly to get to Sam Winchester who was a patient there after coming close to going insane due to his wall breaking and hallucinating Lucifer. Marcus didn't reveal himself as a demon at first, acting as a normal orderly who interacted with Sam several times. At one point, Marcus informed Sam of Marin's past when asked. Marin later stole a lighter from him when Sam needed one.
After Sam damaged his room, in reality putting to rest the vengeful spirit Marin's brother, Marcus was sent to take him to electro-shock therapy. At that point, Marcus revealed himself to be a demon and started torturing Sam using the ECT machine, something he greatly enjoyed. As the machine wasn't having the effect on Sam he wanted it to, Marcus decided to turn the power up even more. As Marcus turned to do so, Castiel appeared in front of him and smote Marcus, killing the demon and saving Sam. Castiel then turned off the ECT machine, ending Marcus' torture of Sam.
- The Demon Marcus may have been working with the four demons that were hanging outside the hospital's emergency entrance.