Marco Gastineau was a member of the San Francisco cult known as Marie's Children. This was inferred in episode eight, when Frank Lanza was remembering previous meetings with Marie's Children member First Born who was accompanied by Tricky Dick and Marco "Machete" Gastineau. Also, Marco was in league with Sucka who was the attacker at scene of Kyle Souller's murder.
While Sucka was stealthily making his was to kill Souller in his home, Marco watched Souller come out of the apartment window and run to his car. Marco alerted Sucka via cell phone that Souller was escaping and then Marco ran over to Souller and tried to hack him with his machete. Souller was able to dodge the blow, which brought him and Marco close together. Souller kneed Marco twice in the testicles hard enough to lift him off the ground and turned to run when Sucka attacked him. After killing Souller, Sucka heard the approaching police cars and tried to get Marco to move. When Marco didn't respond Sucka grabbed Marco's machete, made the protection symbol on Souller's car then made his getaway. Homicide Detective "Pookie" Chang briefly questioned Marco at the murder scene and as he was being taken to the ambulance Bryan Clauser looked at him and felt a powerful and peaceful warmth come over him. When Marco looked at Bryan he displayed an expression of shock, but this was not explained further.
Pookie and Bryan traveled to the Western Edition to talk to Marco Gastineau who was the only witness of Souller's murder. They intended to find out what he saw or find out if he was directly involved. When Gastineau came to the door Bryan felt a strange sensation, a warmth he only ever felt once before, when he had seen Gastineau at the scene of Souller's murder. Before Chang could convince Gastineau to come downtown the air hissed with the sound of a striking arrow, which buried itself deep in Gastineau's heart. Bryan turned and four stories up he saw a glimpse of a green hood and a curved bow. He drew his weapon and sprinted across the street in pursuit leaving Pookie to deal with the collapsed Gastineau.
The author refers to this character with the full name of Marco Gastineau. Within the mob circles he was known with the nickname of Marco "Machete" which referred to his weapon of choice that he carried with him, a machete.
Marco is described by Pookie as being a Caucasian man standing 6 feet 1 inch (185.42cm) tall and weighing 240 pounds (108.86kgs). A description of Marco was also given by Kyle Souller as he was being attacked. Marco's description was given by Kyle Souller as he was being attacked. He was described as having a full black beard with a beer gut. He wore a green ball cap with a yellow John Deere logo and a white tank top shirt or wifebeater. The shirt's nickname, wifebeater, originates from frequent depictions in popular culture of aggressive American men, usually living in poor conditions and having low income, beating their wives and/or children while clad in this shirt
Impressions of Marco's personality were given by Souller before his murder. Souller described Marco as looking like a trailer trash psycho. This is a derogatory North American English term for people who live in trailers or mobile homes, especially in trailer parks. The term is also applied to lower-class white persons in general, as a stereotype, regardless of where they actually live.
From his brief meeting with Bryan and Pookie Marco displayed a defensive and defiant attitude. He showed little respect for the police detectives and referred to them as pigs when they questioned him on the murder of Kyle Souller.