Before the events of the game, Monkey was known as good mime that liked to make people laugh. However being made fun of, he eventually grew tired of the task and decided to become a scientist and do evil, with his intellect being boosted by the Moonstone Shard, making him the most intelligent creature ever, being capable of constructing Castle Grizzlestein.
After Kutaro steals the Calibrus and defeats General Tiger, Monkey reports the subsequent defeat of all the other generals to the Moon Bear King. He appears in the Coliseum when Kutaro faces General Bull serving as the referee of the match without any party knowing his true identity and even rewarding Kutaro with a champion belt. After the defeat of Bull, he puts his plan into motion as he waits for Kutaro in Halloween Ville which he has made into his own lair by converting the Mayor's house into, a massively protected laboratory with several technological traps and devices, while wreaking havoc on the area by transforming the local pumpkins into desserts, and placing General Dog as the guardian of the entrance to prevent anybody from escaping their fate. After Kutaro defeats Dog, Kutaro and Pikarina find a house made of sweets, which is too tempting for them. This gives Monkey the chance to trigger the trap on Kutaro's champion belt which had hypnotic gas, which allowed him to trap the pair into the sweet house, with the desserts being too irresistible for the duo, causing them to fat considerably, while he himself stole the Moonshard Pieces to experiment with them.
After Kutaro and Pikarina thin themselves, they go in pursuit of Monkey at the Mayor's house only to find General Dog transformed, while Monkey watches from afar. After Dog was defeated a second time, the Moon Bear King was incredibly displeased with Monkey's failure expecting more from the general, prompting the tyrant to grab him and nearly choke him, but releases him when the general manages to reveal to his master that Ezma Potts was the one "pulling Kutaro's strings".
Monkey appears at the Land of Time after Kutaro defeats General Rooster. Before Kutaro and Picarina can release Mr. Pink from the Moon Clock, Monkey appears and forcefully places Mr. Pink who serves as the clock's hand to point at twelve, the darkest hour that would open a gateway between Moon and Earth, allowing the Moon Bear King to steal the souls of every kid to grant himself incredible powers.
Monkey is pursued by Kutaro and Pikarina, who are in search of his Moonstone. Monkey hides away in his volcano base, and when cornered he takes to space in a personal rocket. Ying-Yang flies into space and rescues the stranded pair of Kutaro and Pikarina, using Ezma Potts fork to fly the duo to a nearby asteroid. They then chase Monkey who tries to evade them by hurling bombs and missiles at the two. Eventually, Monkey is defeated when Kutaro and Pikarina catch up to him and blast him with the power of constellations.