Mac Rosario immigrated to Japan by boat as a young adult. In Japan, he met Maya while she worked as a waitress. He pursued a career as a Megalo Box fighter before retiring to become a police officer, seeking stability.

His life took a dramatic turn when he intervened in a gang incident, taking a stray bullet that damaged his spinal cord. This caused complete paralysis and left him comatose. Awakening fully paralyzed, he was approached by Sakuma, a researcher from ROSCO corporation, to participate in experimental research using advanced neural-linked exoskeletal medical gear designed to restore mobility.

After extensive rehabilitation and training with the experimental gear, Mac regained full control of his body. He returned to Megalo Boxing directly because of ROSCO's intervention, expressing gratitude to Sakuma and aiming to prove the medical gear's effectiveness. His comeback served as living proof of the gear's potential.

Mac's motivation for returning centered on letting his son witness his capabilities as a fighter. He competed under the ROSCO-affiliated Shirato Group. His boxing style evolved from his earlier emotional and violent tendencies; after paralysis and recovery, he fought with more controlled power, though opponents noted predictable strike patterns.

During the Megalonia championship, Mac faced "Gearless" Joe. This fight revealed Mac's reliance on the gear's enhancements, contrasting with Joe's gearless approach. Despite the technology, Mac's performance demonstrated his regained physical prowess and determination. His journey from paralysis to championship competition highlighted the intersection of medical technology and athletic ambition.