The Ultimate Universe was created with the intent of giving characters a new, modern take. Iconic characters like Spider-Man, The Avengers, and more received books detailing their new origin stories. Some characters, however, did not get the same chance, including Marc Spector's Moon Knight. Ultimate Moon Knight only appeared in Ultimate Spider-Man for a short period, but during this time we could see that his character differed from the 616 counterpart in some key ways, including the exclusion of one of Marc's personalities.
Marc Spector in the ultimate universe was an ex-Navy Seal who was a part of a super-soldier program that went wrong during the Gulf War. He later began working for Donald Roxxon as a mercenary under the name Paladin and began operating as Moon Knight during this time as well. It is unknown if this backstory is true, however, as Marc possesses severe dissociative identity disorder, having at least four unique personalities that blend together. These are Marc Spector, Steven Grant, Moon Knight, and Inner Child. These personalities are frequently seen interacting with one another in a mental plane. It is unknown if Marc Spector or Steven Grant is the dominant alias within this universe.
One major distinction that makes Ultimate Moon Knight unique is the omission of Khonshu in his origin story. Whereas 616 Moon Knight's powers are mystical in nature, this Moon Knight's powers seemed to be derived from the super-soldier serum. His reasons for fighting crime in the Ultimate Universe are unknown, however, it is seemingly tied to his DID. Moon Knight was a unique personality within Marc's (Or Steven's) mind. Ultimate Moon Knight was also able to create new personalities, as he was seen forcibly creating the Ronin persona to infiltrate the Kingpin's crime ring. These personalities were also able to kill one another, as Ronin is seen killing Moon Knight in Marc's mind.
Another exclusion was one of Marc's key personalities, Jake Lockley, who was instead replaced by a particularly interesting character, a red-haired little girl known as Inner Child. She was never seen physically taking over the body, however, she seemed to operate as a voice of reason between the other personalities. She was seen strongly opposing the creation of the Ronin persona as she feared it would become the dominant personality. Her concerns go unheard, but later prove to be justified as Ronin goes to extreme measures to prove his loyalty to the Kingpin. Elements of Jake's character can be seen in the Ultimate version of Steven Grant, who was a cab driver and operates at the street level.
The character plays a large role in bringing down the Kingpin. After infiltrating his inner circle as Ronin, Kingpin discovered that Ronin is Moon Knight masquerading as a killer. After this he shoots Ronin in the head, ordering his men to dispose of the body in a river. Despite this, Marc survived and reported Fisk to the police, leading to his downfall.
Ultimate Moon Knight's distinctions make for a compelling modernization of the character. As opposed to the current version of Moon Knight, he is more grounded in reality, with his multiple personalities playing an even larger role. With season 2 of the MCU Moon Knight show seemingly on the way, one has to wonder if the show will adapt elements of the Ultimate Universe version of the character.