Mark is the former lover and boyfriend of Lewis Ha.
In "The Open-Ended Nature of Unwitnessed Deaths", Phil tries to apologize to Lewis for accusing him for what happened to the building's power, but discover that Lewis is depressed as it would've been his anniversary with Mark. Despite Lewis' objection that Mark is probably dead, Phil continues to believe the small possibility that Mark is alive. Phil then tries to convince Lewis to go with him to Seattle, Washington to go to Lewis' old house to see if Mark is there. Lewis ultimately rejects but is tricked by Phil into going.
Lewis becomes ultimately pissed off, but Phil insists that they leave a note just in case that Mark is alive. Lewis still pleads that Mark is dead. Phil then places a paper bag over his head as he thinks Lewis will become attracted to him as he "looks like Mark". They then arrive at Lewis' House. Lewis becomes happy to see old photos of him and Mark. He begins to cry and hears the floorboards creak, believing it to be Mark, but it's actually Phil. Lewis becomes angry and kicks Phil out of his house, but is invited back into the house. Before leaving, Lewis decides to leave a note anyway for Mark.