Gainer Sanga is the central protagonist of the story, a seemingly ordinary high school student living in the Siberian Domepolis. He is a renowned champion of virtual combat games, a skill that has earned him the title of King Gainer after achieving his two hundredth consecutive victory in a tournament. Despite his prowess in the digital world, Gainer is a pacifistic and reluctant hero who does not enjoy real fighting and is keenly aware of the difference between video game simulations and the harshness of reality. His personality is initially marked by a strong skepticism and resistance to the Exodus movement, a forbidden mass emigration from the domed cities, and he makes several attempts to refuse the call to action. He is often portrayed as a grumpy bear or a punch clock hero, someone who ends up fighting for a cause he did not originally believe in.

Gainer's background as a champion gamer directly informs his role in the story. When he is arbitrarily arrested on suspicion of being involved with the Exodus, he finds himself in a cell with Gain Bijou, a notorious and experienced Exodus specialist. Together, they break out of prison and infiltrate a private museum to steal a highly advanced and mysterious mecha known as an Overman. Gainer becomes the pilot of this machine, which he names King Gainer after his gamer alias, and his exceptional video game skills translate surprisingly well to piloting the real robot, allowing him to quickly master its controls and combat abilities. The initial objective of the Exodus is to lead a massive caravan of mobile cities from the Domepolis to the ancestral land of Yapan.

Key relationships significantly shape Gainer's development throughout the narrative. His most important relationship is with Sara Kodama, his strong-willed classmate and childhood friend who is an active member of the pro-Exodus group. Their dynamic is fraught with misunderstandings and flip-flopping emotions, but after a long journey and the defeat of a major antagonist, they eventually confess their love and come together as a couple. He also shares a close but strictly platonic life partnership with another pilot named Cynthia. His relationship with Gain Bijou is a classic dynamic between a junior hero and his mentor, with Gain possessing years of real combat experience that contrast with Gainer's simulation-based skills. A significant emotional trial for Gainer occurs when he discovers that a man named Gauli was responsible for killing his parents. Rather than seeking revenge, Gainer chooses to turn the other cheek, recognizing that retribution would be a pointless act.

Gainer's personal journey is one of maturation and reluctant acceptance. He begins as a reluctant participant, actively trying to avoid the responsibilities thrust upon him. However, through his forced involvement in the Exodus, he grows into a more mature and capable individual. A major turning point in his development involves the Overdevil, a powerful and malevolent entity. When controlled by the Overdevil, Gainer is brainwashed and becomes an ice person, a version of himself that is emotionally cold and terrifying, with his signature glasses becoming menacing and shiny. This experience, from which he is eventually freed, represents a profound loss of self that he must overcome. As a pilot, Gainer's notable ability is his exceptional talent for controlling the Overman, a skill born not from formal training but from countless hours of playing video games, making him a prodigy who learns and improves even while fighting.