“Who is next? Who has the guts to step into the ring with Little Yama?”
―Mr. Yama arrogantly calling for another contestant
Mr. Yama is first introduced as the undisputed bot-fighter champion in San Fransokyo's underground bot fights with his prize robot, Little Yama. After defeating an unfortunate contender, a boy named Hiro Hamada shows up and asks if he can participate with his own robot. Yama, underwhelmed by Hiro's naive demeanor, agrees to fight the boy. When Hiro's bot seems to collapse after the first round, Yama is about to gloat in his victory until Hiro bribes him with more money for a rematch. The confident Yama accepts this and the match restarts.
However, Hiro's bot is revealed to be highly advanced, quickly repairing itself and destroying Little Yama with ease. Hiro brazenly boasts about his victory until Yama, out of spite, has his goons attack the boy and chase him into the alley. When the group seems to have Hiro cornered, Hiro's brother Tadashi rescues him on his moped away from the goons.
The brothers, along with Yama's gang and everyone in the bot fights, are caught and arrested by the San Fransokyo police during the escape. Yama and his gang are forced to share a cramped cell with Tadashi, while Hiro has a separate cell to himself due to being a minor. Hiro and Tadashi are released that same night on account of their aunt, while Yama and his henchmen are presumably left in the police station.
Yama is the main antagonist of the video game, Battle in the Bay, where he sought revenge against Hiro for defeating him in the bot-fighting tournament. To do so, Yama invaded San Fransokyo with an army of battle bots, setting them against the Big Hero 6 team. After a battle on the Golden Gate Bridge, however, Yama is defeated and sent back to prison.
In earlier drafts, Yama was intended to be a more important antagonist as a mob boss in San Fransokyo's crime scene. He was the leader of a gang, which consisted of rocket-powered bomber men, a sumo wrestler, and the Fujitas; a group of martial artist roller women in traditional Japanese wear and makeup.