Marco Owen, a muscular man covered in intimidating tattoos, was incarcerated for hacking CIA and Venus Gate computers, resulting in a sentence of over half a century with 60 years remaining. The NSA offered him parole to infiltrate Venus Gate's facility, investigating the Medusa virus and gathering evidence on the secretive Level 4 labs, while he simultaneously pursued a personal vendetta against Zeus—a former friend who framed him.

Posing as a cryogenic treatment patient, he entered the facility. During the Medusa outbreak and ensuing chaos, he was knocked unconscious and later presented himself as a survivor. His appearance and mannerisms aroused suspicion, especially after the group discovered he avoided Medusa infection. Despite this, his combat skills, leadership, and resourcefulness led the survivors to tacitly accept him as their de facto escape leader.

He first encountered Alice’s ghostly apparition and frequently argued with her. Viewing security footage linking Kasumi Ishiki’s twin sister Shizuku to Level 4’s monstrous entities, he grew suspicious of Kasumi. His primary focus remained locating Zeus, often distancing himself from the group.

In the manga’s climax, Zeus’s genetically engineered soldiers killed him. Alice resurrected him by sacrificing her life, granting him temporary Medusa-derived abilities. He confronted Zeus directly, hacking into his system to erase him permanently. Owen’s ultimate fate alongside Kasumi is ambiguous, with implied mutual romantic feelings.

The film adaptation diverges: he dies from battle injuries after extracting Kasumi’s promise to live fully and does not return. Across both media, he evolves from a mission-driven loner to prioritizing others’ protection, particularly Kasumi’s. His backstory reveals layers beyond his criminal facade, including hacking expertise and the betrayal shaping his motivations.