Before the other alters were formed, there was only Marc Spector. The world knows him as a dangerous ex-military man who went AWOL, killing innocents for money. In reality, Spector is a man with a broken past, of which Khonshu takes advantage of and recreates him as one of the world's most fiercest avenger for justice.
He's driven by enormous amounts of guilt, and pushes people close to him away, including his wife. This leaves him to deal with the duties of the Moon Knight alone, without any emotional support of any kind. When asked by other avatars of his condition, his only response was a desperate "I need help."
Before Steven knew who he was or how they were related, Marc restricted him from ever figuring out their past or secret superhero life. This was due to his wish for Steven to have the closest resemblance to a normal life, as well as to keep him safe. So imagine Steven's surprise when he first learned of Marc's existence, when he sees 'himself' in the cameras, wrecking absolute havoc in a public place.
Towards the end, Marc begins to befriend and rely on Steven, forming almost a brotherly bond, sharing their responsibilities and burdens as Moon Knight, as well as their memories of Mother's abuse. Steven becomes a pivotal companion for Marc to rely his past and distress with.
Besides the sheer amount of angst within Marc's character, he brings entertainment in scenes, especially with his interactions with Steven. His complexity brings an interesting story to his role as Moon Knight.
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