Marco, also known as Marco the Phoenix, is the former first division commander of the Whitebeard Pirates and served as the crew's doctor and closest confidant to his captain, Edward Newgate. He first joined the Whitebeard Pirates as an apprentice at a very young age, around fifteen when the crew visited Wano, and over decades rose to become the most trusted subordinate, effectively acting as the second-in-command and right-hand man of the entire fleet. After the death of Whitebeard during the Paramount War at Marineford, Marco briefly assumed leadership of the remnants of the Whitebeard Pirates until they were decisively defeated in the Payback War against the Blackbeard Pirates, after which he retreated to Whitebeard's home island of Sphinx and lived in relative isolation, working as a doctor for the local villagers.

Marco's personality is defined by a calm, steady, and collected demeanor. He is remarkably levelheaded in crisis situations, often acting as a mediator during conflicts and providing stable leadership when chaos threatens the crew. Despite his generally serious and responsible nature, he also displays a casual, laid-back attitude and is known for a dry, sarcastic sense of humor, often making witty remarks during tense moments. He is deeply loyal and protective, placing the well-being of his captain and crew above all else, and he does not seek personal glory or recognition, finding fulfillment in serving Whitebeard's vision and supporting his comrades. His core motivations are driven by a profound sense of duty and family; he views the Whitebeard Pirates as his family and is dedicated to preserving and honoring Whitebeard's legacy, especially after his captain's death. This commitment ultimately leads him to support the next generation of pirates, particularly Monkey D. Luffy, whom he recognizes as carrying the same spirit of freedom and genuine care for comrades that defined Whitebeard.

Marco's role in the story is significant across several major arcs. He was instrumental during the Marineford War, where he fought simultaneously against three of the Marine's admirals—Kizaru, Akainu, and Aokiji—in a desperate attempt to protect Whitebeard and rescue Ace. His efforts were a central part of the Whitebeard Pirates' offensive. After Whitebeard's death and the defeat by Blackbeard, Marco's role shifted to that of a guardian of memory and a reluctant elder statesman, living quietly on Sphinx Island. He later reemerged as a crucial ally during the Wano Country arc, where he traveled to defend the samurai and the Straw Hat Pirates against the Beasts Pirates. He played a pivotal role in the raid on Onigashima, single-handedly holding off two of Kaido's strongest subordinates, King and Queen, for an extended period while also using his healing flames to slow the spread of Queen's Ice Oni virus and support the allied forces.

Marco's key relationships are deeply rooted in the concept of family. His bond with Whitebeard is the most defining relationship of his life; Whitebeard was a father figure and mentor, and Marco's loyalty and devotion to him were absolute. He was also close with the other division commanders, maintaining strong camaraderie with Jozu and Vista, and shared a particularly close friendship with Portgas D. Ace. Initially mistrustful of Shanks, Marco grew to respect him and later allied with the Straw Hat Pirates, especially Luffy, whom he assisted directly in Wano out of respect for Ace and a belief in Luffy's cause. His relationship with Luffy represents his conscious decision to continue Whitebeard's legacy by supporting the next generation of pirates who fight for liberation.

Marco undergoes notable character development throughout the series. His journey traces a trajectory from an active, frontline warrior and loyal subordinate to a contemplative elder statesman bearing the memory and legacy of his fallen captain. The loss of Whitebeard and the destruction of his crew forced him to process profound grief, which he converted into a quiet, continued service to his captain's ideals rather than seeking revenge or despair. His decision to leave his self-imposed isolation on Sphinx to fight for Luffy in Wano, despite being weakened and emotionally exhausted, marks a significant turning point. This act demonstrates his growth from a man defined by his past loyalty to one who actively chooses to protect and foster the future, embracing the role of a mentor and guardian for the next generation.

Marco's most notable abilities come from his Devil Fruit, the Tori Tori no Mi, Model: Phoenix, a rare and exceptionally powerful Mythical Zoan-type fruit. This fruit allows him to transform into a phoenix and a human-phoenix hybrid at will, granting him full flight capabilities and enhanced physical strength. His signature power is the generation of blue flames of regeneration, which grant him near-limitless regenerative abilities, allowing him to heal from almost any wound instantly, making him extraordinarily difficult to kill. These flames are not destructive like normal fire; instead, they are flames of life that can be used to heal his own injuries and, to a lesser extent, the wounds of others, providing invaluable battlefield support. He can also weaponize his flames in solid constructs for offense and defense, such as powerful kicks called Phoenix Brand and defensive shields called Immortal Thistle. Beyond his Devil Fruit, Marco is a highly skilled combatant proficient in Armament Haki, which he used to land hits on Logia-type admirals, and Observation Haki, which he demonstrated by sensing Luffy's awakening from a distance. Despite his immense power, his abilities are not without limitation; prolonged use and damage from Haki-infused attacks drain his stamina, and he is still vulnerable to Seastone, which negates Devil Fruit powers.