From Halopedia, the Halo wiki
On December 12, 2553, Marco and two others were with their boss, Ross Nyeto, at the Trattoria Georgi in New Tyne, Venezia. The four were approached by Office of Naval Intelligence operative Veta Lopis, who was undercover as a thief looking to fence stolen United Nations Space Command ordnance. When Veta revealed she knew just who his boss was, Marco and the others tensed and motioned toward their weapons under the table they were seated at. Veta pretended to be looking to rent the Ghost Flag, a Razor-class prowler, from Nyeto, and to make this point she showed off a number of boson beads, to Marco's astonishment. He asked where she had come by the phase pearls, but she refused to say. Ross was reluctant to make a deal with her. She mad to leave the table, but Marco slapped a hand over hers as she was gathering the phase pearls and told her she could leave them right where they were. At this, she first grabbed his thumb and then slipped into a wrist lock, employing a pain-compliance technique she had learned during her years with the Gao Ministry of Protection. Marco helplessly flopped across his boss' lap. He bellowed in pain when she lifted his arm ever so slightly. The sudden violence caused all eyes in the dining room to focus on the scene and both Nyeto's other men and the three disguised Spartan-IIIs with Veta to draw their weapons. Just then, the owner of the establishment, Georgi Baklanov, emerged from his office and deescalated the situation. Veta let Marco go and Ross apologized for him. Before long he was part of the ongoing conversation as if nothing had happenned. When Blue Team finally breached the restaurant as had been the plan all along, Marco was killed.
Personality and traits
Marco was brash and showed little regard for common decency, as evidenced by his willingness to just keep the boson beads after being shown them. He was a head shorter than his boss but almost as wide (Ross Nyetto had shoulders as wide as a door). He was moonfaced and had beefy hands, rough skin, and slit eyes. On the night he was killed, he wore a ballistic vest over his bare torso, a popular style that afforded more "bad-boy fashion" than it did effective protection, according to Lopis. He also carried a New Tyne Armory 12mm Comet sidearm.
List of appearances