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|The 4400 character|
|First appearance||"The New and Improved Carl Morrissey"|
|Last appearance||"The Great Leap Forward"|
|Created by||René Echevarria
|Portrayed by||Richard Kahan|
Marco is in charge of NTAC's theory room, and is the one who initially proposed that each 4400 has caused a "ripple effect". He often appears to somewhat resent his colleagues, at one point labeling them as the "Two most annoying people on the planet".
His relationship with the NTAC agents Tom Baldwin and Diana Skouris is strained at first; Tom had little patience for the Theory Room's methodology, and Diana claims that he intimidates Marco and his colleagues. When Tom prepares to rescue his son Kyle, Diana goes to Marco for help. At this point, Marco hints at his feelings toward Diana, claiming that he isn't helping for Tom's benefit; the look of shock on Diana's face implies that she had been completely unaware of his feelings.
Marco continues to aid the NTAC agents from the Theory Room. His friendship with Diana grows, with her teasing him affectionately. He makes a forgery of Diana's daughter Maia Skouris' vision diary, so that Diana will not have to hand the real one to the government for analysis. Maia had even predicted Marco's future death in her diary. Although surprised when he read about it, he thought that the manner of his foretold death was "actually pretty cool."
Marco and Diana finally begin a relationship during the season 2 finale, "Mommy's Bosses," as Marco says he will support Diana through the long process of bringing the promicin inhibitor conspirators to trial.
In the season 3 episode "Blink," Diana ends the relationship with Marco as she believes she is using him to avoid becoming close to a man more her type. Despite being hurt at the rejection, Marco is never shown to be anything other than supportive of her. Whilst dating Diana, Marco had become friendly with her daughter Maia, and Maia asks after him even after the breakup.
Marco informs Diana that April is missing and in possession of promicin, leading to Diana's reinstatement into NTAC and move back.
He later believes that making conspiracy movies is the ability of 4400 Curtis Peck. When Diana is searching for a safe place to hide Peck, Marco uses his apartment (leading to a momentary awkwardness as he and Diana remember their brief relationship) as a base for Peck to write his newest script, although Peck is later compromised by "The Marked".
When Marco's colleague uses his own promicin-induced ability, putting many of the principal "players" into a game of survival, the failed relationship between Marco and Diana is re-examined. Maia does not understand why Marco no longer visits them, and Marco admits to Diana that he does find it a little hard seeing her every day. Diana asks if he would be willing to give genuine friendship a try, and offers him an invitation to dinner with her and Maia.
After being infected by a promicin-inducing virus in "The Great Leap Forward", Marco can seemingly teleport to any place he is thinking of, as he appears in Promise City after looking at a photograph of the location.