Marco stands as the lone surviving servant of House Granatum after a brutal attack claimed Astolfo Granatum's family. He accompanies Astolfo when the latter joins the Chasseurs, a church-affiliated vampire-hunting organization, becoming Astolfo's vice-captain. Marco's unwavering loyalty persists despite visible fear during violent operations and personal discomfort with their brutal methods.

Physically, Marco is a tall adult man with short blond hair slicked back, a prominent nose, and square glasses. He wears the standard Chasseur uniform: black pants, a cassock, a pale harness, a rosary, and a white cloak.

His defining trait is absolute devotion to Astolfo, fueled by guilt over surviving the Granatum massacre while failing to protect Astolfo's father. This motivates his declaration: "On the day the mansion was attacked, I escaped by myself and hid. I was unable to protect my master, your father. This time, I won’t let you die! I won’t let you die, Young Master!"

Marco actively participates in field missions, notably during the Beast of Gévaudan investigation where the Chasseurs aim to suppress historical truths. He voices concern over Astolfo’s solitary actions, exemplified when locating him alone in a cave and pleading, "I begged him to refrain from acting on his own." He later provides tactical support during Astolfo’s fight against Noé Archiviste at the witch’s castle, reinforcing Astolfo’s abilities despite the latter overexerting himself and requiring rescue by Roland Fortis.

Within the Chasseurs, his role involves operational assistance rather than direct combat leadership. He is frequently depicted reacting to Astolfo’s decisions with anxiety or attempting to mitigate risks, reflecting a dynamic of protective subservience.

Marco appears across manga chapters (Mémoire 23, 25–26, 28, 30, 33–35, 38–42) and anime episodes (Mémoire 14–19), consistently alongside Astolfo.