Marco is a loyal high ranking commander in Whitebeard's Pirate Crew.
Marco is the commander of the First Division in the Whitebeard Pirates, and as such he has authority over most of the lower ranking members of this corp. Even with all this responsibility he has a laid back easy going attitude.
Marco ate a Mythical Zoan type devil fruit, which as of now is the rarest known type of devil fruit powers, which gives him the ability to transform his body into a Phoenix. Because of this he has incredible powers including the ability to spread flames, fly, and survive or even come back from fatal blows, making him almost immortal.
During the Marineford Arc, Marco was present with the rest of the Whitebeard Pirates when they stormed Marineford to save Ace. During the fight he shows off his powers for the first time and we see that he is strong enough that he can even fight and damage all three of the Admirals despite their Logia powers.