Marco was once a test subject for Professor Hojo who turned him into a "Sephiroth-clone", a person with Jenova cells whose own psyche has been weakened to the point Sephiroth can exert his will through him. At some point, he moved to the Sector 7 slums and became Tifa's neighbor at Stargazer Heights. He seems unable to communicate much and groans constantly, as if in pain, and Tifa and the others take him to be very ill.
In "Home Sweet Slum", Marco stumbled outside after Cloud knocked on his apartment door, having heard sounds of anguish from inside. Cloud saw him as Sephiroth and proceeded to attack, but Tifa stopped him and Cloud snapped out of the illusion. She informed Cloud of Marco's "few problems", assuring he wasn't a "bad guy", and that Marle had her check on him; she asked Cloud to do the same, who reluctantly agreed.
Marco later appeared at the Shinra Building in "Deliverance from Chaos", again taking on the form of Sephiroth. As Sephiroth, he killed President Shinra and Barret (who was resurrected by the Whispers), transformed into Jenova Dreamweaver, fought the party, and was defeated and disappeared. Two Sephiroth-clones appeared at the building, one Palmer spotted strolling through the corridors, and one who also faced Cloud outside Jenova's tank and freed her, carrying her away. Whether the "Sephiroth" Palmer saw was also Marco is unknown, but the Sephiroth who escapes the building with Jenova was clone #2.
Marco is a man with pale skin, sunken features, and a bony physique. He wears a dark tattered robe and has a black '49' tattooed on his bicep. The robe has long fluttering pieces of cloth at the back, something like a cape or invoking the image of wings, that have a white symbol painted on them.
Marco is a Sephiroth-clone, thus Sephiroth controls his will. Tifa tells Cloud that he has a "few problems", but isn't a "bad guy". What Marco was like before his transformation into Sephiroth's thrall is unknown.
Etymology and symbolism
Mars (Ares in Greek mythology). The name is also popular in English speaking countries as "Mark".Marco is an Italian male given name derived from the name of the Roman god of war,
Marco's tattoo is 49, much higher than the Sephiroth-clones' numbering system reached in the original Final Fantasy VII. It could derive from 7x7=49, the number 7 being a recurring theme in the Final Fantasy VII games, and also perhaps because 4 and 9 are considered unlucky numbers in Japanese superstition: 4 is pronounced like shi (death) and 9 is pronounced like ku (suffering). It could also refer to the Buddhist concept of bardo, the intermediate stage between death and rebirth, which according to some interpretations can last for up to seven weeks (49 days). The robed men act as hosts to the true Sephiroth, who posthumously affects the mortal world by acting through them.