He is the second main character, and by far the most calm. He is revealed to be a criminal who had gotten into trouble with the CIA by hacking into their database for reasons unknown. He has many layers to him, and seems to know a good deal more than the rest of them do.

Despite his criminal history, Marco appears to care very much about what happens to the other survivors, regardless of the fact that none of them have more than six weeks to live. He has regularly put his life on the line without a second thought in order to save the others, even those who resent him. It also arguable that although he has certainly not been upfront with information, he has never actually flat-out lied to any of the survivors.

Marco is also the unofficial leader of the group, partially due to his knowledge and partially due to his natural capabilities. While in prison, Marco built up his muscles, received his tattoos, and acquired a new attitude that makes him come off as more of a thug than a hacker -- so much so that his intelligence with computers surprises Katherine and Kasumi, despite already knowing who Marco is.

(Source: Wikipedia)