But then, in the second Moon Knight series from 1985, she ends up dumping Marc because he's started seeing ominous visions of Khonshu and is having trouble keeping track of who he is. From then on, Marvel writers increasingly wrote Marc as being haunted by the rich guy, the cabbie, and the superhero living in his head. He even quits Moon Knighting for a while because the stress of being four people is too much for him.
Marvel Comics
At one point, the cabbie personality takes over and makes him move to Mexico and join clandestine fights for money. In 2011, Marc finally ditched those other personalities ... only to replace them with Wolverine, Spider-Man, and Captain America. He even starts dressing up like combined versions of those guys.
Marvel Comics
Up to this point, the implication was that Marc developed multiple personalities by pretending to be other people for too long during his missions. In his 2014 series, Marc finally gets around to seeing a psychologist, who tells him that this idea is ridiculous -- what actually happened is that a god from another dimension invaded his mind and gave him brain damage (which is probably a way more common diagnosis in the Marvel Universe). So, basically, Marc isn't really insane; it's all Khonshu's fault.
Marvel Comics
Marvel Comics
This was contradicted a couple of years later when Marc remembers that he developed his Steven Grant and Jake Lockley personalities when he was a kid, way before Khonshu supposedly brought him back to life and messed with his brain. It's explained that this happened due to the trauma of finding out that a family friend was secretly a Nazi war criminal and having to keep the secret. Also, the whole reason he became a mercenary was that he was kicked out of the Marines after they found out about his mental health history. But, double plot twist: turns out this was all orchestrated by Khonshu, who is definitely real and had been grooming him as a potential host since he was little, the massive creep.