Moon Knight was an ex-Navy Seal who was in a Gulf War Super Soldier experiment that went wrong. He later worked for Roxxon under the name of Paladin until Roxxon fell apart.
He seemed to have fragmented psychosis and had developed four personalities including Marc Spector, Steven Grant, Moon Knight, and a red-haired little girl. He lived with a lady named Marlene who was his girlfriend at the time. She often complained about his different personalities.
Moon Knight attacked Spider-Man when he mistakenly believed he had blown up a near by building.. This battle with Spider-Man put him at grudging odds with the younger hero in future encounters. Later, after videotaping Captain De Wolfe in China Town as Steven Grant, he went out as Moon Knight only to engage in the gang war waged by the Kingpin and the newcomer Hammerhead. During his battle with Spider-Man, Iron Fist, and Shang Chi he was impaled with a sai to the chest by the assassin Elektra. Right before falling out into a coma, he impaled Elektra with a moon-blade into the back of her head before she could kill the Black Cat. Once he woke up, Moon Knight escaped from custody and engaged in another fight with the Punisher, Spider-Man, and Daredevil.
After the battle, Daredevil invited Moon Knight to join an organization of super-heroes with the goal of bringing down the Kingpin. As part of Daredevil's team, Spector adopted the identity of Ronin to infiltrate the Kingpin while acting as a double agent.
The Kingpin told Ronin to prove himself by retrieving Spider-Man. Ronin drove a school bus into Midtown High School and started firing a gun to get Spider-Man to fight. Kitty Pryde, who had recently enrolled there as a student, confronted him, but was knocked unconscious shortly before Spider-Man arrived. The two engaged in a brutal battle that was mirrored in a fight between Moon Knight and Ronin in his mind (since they were two different personalities within the same person); Ronin knocked Spider-Man unconscious in the end. The Moon Knight personality was apparently killed. Ronin used Spider-Man's own web-shooters to leave the high school, and took him to the Kingpin, who interrogated the hero at his hideout.
After Iron Fist betrayed the team only to save his daughter, the Kingpin discovered that Ronin was working for Daredevil and kept that knowledge until after Ronin captured Spider-Man. Ronin was then present in the Kingpin's interrogation with a unmasked Peter Parker, and was then revealed of his identity when the Kingpin brutally beaten Ronin along with Peter. The Kingpin then ordered Ronin's execution. Kingpin's men took Ronin by a river and shot him in the head. However, he regained consciousness, and survived along with his Moon Knight personality. He went to the police claiming that the Kingpin attempted to murder him. This provided a charge for the police to arrest the Kingpin, but he had to reveal his secret identity for a charge to be placed. The public called him a hero for his efforts.
None known, though he was a Super Soldier, and was faster than Spider-Man, may have possessed some level of enhanced strength, even before Doctor Strange's mystical enhancements (such as when he broke a brick wall by kicking it in Ultimate Spider-Man #80), and may have possessed some enhanced healing, possibly based around his MPD (such as when he healed from a coma in Ultimate Spider-Man Annual #2 and when he healed from a headshot in Ultimate Spider-Man #110, both due to his various personalities telling him to "wake up")
Moon Knight is an exceptional athlete, acrobat, and combatant, skilled in various weapons and martial arts, capable of shattering brickwork with a blow. Unlike most MPD sufferers, Grant's personae are aware of each other and their actions, and they usually cooperate to achieve shared goals; inexplicably when Moon Knight has twice been rendered unconscious by severe injuries, the others remained active in his mindscape and proved capable or reviving him. He also has no finger prints capable of identifying him.