Marco Pacella (Richard Kahan) is the lead analyst in NTAC's theory room. Alongside fellow "brains" P.J. and Brady, Marco aids Tom Baldwin and Diana Skouris in their investigations by supplying them with in-depth analysis and technological expertise.
His friendship with Diana slowly blossomed into a romance, but the relationship was short-lived. Diana ended their courtship, convinced the two had no real future and believing they were better off as friends.
Marco has helped solved many of NTAC's most difficult cases and continues to be a valuable asset and a respected member of the team.
Along with Tom and Diana, Marco tries to find the source of a computer virus that is disrupting ATM's and causing computer failures all over the country. Eventually, he is able to trace the disruptions to Warren Trask, a man who was swindled by Drew Imroth, the head of Ubient. Even though on life support, Trask has used his ability to go into the internet and disrupt it from inside.
When Trask is killed, Diana and Marco find evidence implicating Tom as the possible murderer.
Marco awakes to find himself trapped at NTAC with many of his co-workers as well as Collier, Kyle, and Isabelle. Eventually, PJ confesses that they are all trapped because he took promicin and developed the ability to draw others into a game that takes place in a shared psychic space. In the game, the players have to work together to overcome an obstacle; in this case, the obstacle is NTAC itself. When the building kills off Meghan, Shawn, and PJ, Collier and Tom join forces, however briefly, and save everyone by short-circuiting the building.
Marco informs Tom and Diana that a 4400 named Curtis Peck has disappeared. Peck's ability is to use his filmmaking skills—or lack thereof—to reveal the truth about conspiracies. Peck's latest conspiracy involves the Marked, people who've come back from the future and implanted their consciousness into ten prominent people in order to stop Jordan Collier.
Tom comes to Marco with several hairs that he wants tested for promicin. Tom insists that Marco ask no questions and keep the test utterly confidential. A curious Marco returns the results to Tom with the news that the hairs came back positive.
Marco finds out that Diana's sister April was on a list of people who obtained promicin.
Later, Marco hypothesizes that Collier has the ability to take away the abilities of "extra crispies," non-4400's who take Promicin.
SEASONS ONE-THREE TIMELINE
Marco begins to piece together the "big picture" of what the future had in mind when returning The 4400. Everything that the returnees do -- good or bad -- has a ripple effect on future events that, presumably, will result in some final good.
Following the "awakening" of Kevin Burkhoff, Marco proposes that perhaps the brilliant neuroscientist is the one who made the powers of The 4400 possible.
What started out as a professional relationship seems to progress and Marco and Diana begin to spend more time with each other.
Growing closer to Diana, Marco suggests that maybe her daughter, Maia, would be happier in the 4400 Center school, where she would be around children like herself.
Marco assists Diana in her search for the truth about TJ Kim's murder.
Worried about Diana, Marco does everything he can to help find Maia. Working with Diana and Tom, Marco searches Maia's diary for clues regarding her whereabouts.
Marco manages to track down Maia Rutledge, discovering that the future returned her to 1847.
Marco is surprised to learn that Diana was once engaged to Josh. When Diana later calls Marco over to her house he realizes it can't be good news. She intends to end their relationship.
Their relationship officially over, Marco does his best to cordially work with Diana at NTAC.
Marco uncovers the connection between the three murder victims in Tom and Diana's case – they all have something known as the Starzl Mutation. He further assists the investigation when he is able to connect one of Haspel Corp's recruits to the murders. Although Marco is essential in helping the case, he still struggles with his new platonic relationship with Diana.
After finding a journal in one of Jordan Collier's many real estate acquisitions, Marco is put on the task of analyzing the handwriting and content. He confirms that the notebook does in fact belong to Collier and calls NTAC's attention to a phrase written over and over again: "the war for the future will be fought in the past." Marco surmises that the diary is a prophecy.