Marc Spector is a former United States Marine and mercenary with dissociative identity disorder. During his childhood, Spector's brother Randall drowned in a cave while they were playing, starting a strenuous and spiteful relationship with his mother, who frequently abused him. To find comfort within himself, Spector's psyche formed an alter known as Steven Grant, who had no memory of Spector's trauma. While he was on a mission in Egypt, Spector was shot by his partner Raul Bushman, but he was saved by Khonshu, who transformed him into his Avatar and the vigilante Moon Knight. Soon after, he fell in love and married Layla El-Faouly. After his mother's death, the Grant alter resurfaced and relocated to London to become a museum worker, with Spector occasionally taking over the body to operate as Moon Knight.
Marc Spector used to be an incredibly happy person as a child, who was confident, smart, and not afraid to face his fears. However, over time due to his extreme guilt over his brother's death, the extreme childhood neglect and beatings he used to receive from his mother, Spector started to develop a fractured mind, resulting in him developing Disassociative Identity Disorder. Wanting a happy life, and a happy childhood, Spector created his alter, Steven Grant, a socially oblivious man who lived a happy life with a loving mom, in order to guard himself against the pain of his life.
Spector eventually managed to get some form of control over his alters after moving out of his family's house to become a US Marine as soon as he could. However, despite this, due to his extreme trauma his alters continued to show up at some points of his life, which forced him out of the Marines. His mind was so fractured due to his trauma, that it was even acknowledged by the likes of Khonshu, who used his moments of weakness to manipulate him into becoming Moon Knight, the avatar of Khonshu who serves vengeful justice.
Due to having multiple alters, Spector is highly prone to aggression and internally anguished to the point of being driven to tears after being asked by the gods, about his pain due to his alters and Khonshu. As a result of his psychological rifts, Spector became a bitter, rageful, and ultimately lonely person who had developed anger issues over time, as shown when Spector shattered everything around him after Grant started fighting against him. Despite all of his pain he had gone through, Spector is a brave and courageous man who is able to think rationally in high stress situations, as he was able to talk and explain to the panicked Steven what he should do while the latter was being followed by the jackal. Spector has a tendency to be brutal and ruthless with his enemies as he had brutally killed many of Arthur Harrow's men with his hands and weapons, which attributes to him being trained as a mercenary.
As a result of his trauma due to the various events in his life, Spector became a very pessimistic and socially inept man, who always hesitated to communicate his emotions against the world because he always feared the worst out of people, as shown when he was unable to express his feelings of love towards even his own wife Layla. This resulted in him becoming a dishonest person who tried to hide from tough situations, despite having noble intentions. This was demonstrated best when Spector chose to abandon Layla rather than communicating to her about his tough past with his mother, his fractured mind, and his situation with Khonshu, in fear of endangering her or hurting her feelings.
Nevertheless, despite all he had gone through, he continued to possess a strong moral code, such as refusing to kill a child who was part of Arthur Harrow's cult. He does care about the well-being of others such as his wife Layla. Spector displays incredible resilience as he chooses to do the right thing to help others, through continuing to deliver out his justice, regardless of what his actions has given him. Due to having committed extreme acts of violence, under the name of Khonshu's vengeful justice, Spector was deeply pained by the burden that his actions had placed on him, feeling extreme feelings of guilt due to his actions, as shown when he remembered each and every person he was forced to kill as a servant of Khonshu and his vengeance.
Despite managing to gain a form of balance after finding love in the form of his wife, Layla El-Faouly, Spector continued to develop a greater self-hatred for himself over time. When Spector had discovered that the woman that he had hated for being abusive to him - his mother, had died after years of not talking to her, Spector became absolutely devastated as he blamed himself for causing his mother pain, falsely blaming himself for his brother's death. His pain grew to be so severe that Spector was unable to stay in control, which brought Grant out, in order to shelter himself from confronting his mother's death's effect on him. Only this time, his self-hatred grew so big that he lost nearly all control over his multiple alters, making Steven Grant his prominent side for a while.