Monkey is the small pet monkey of Science Guard Party member Pigu. He understands human language and possesses a mischievous, playful nature, typically accompanying Pigu during team activities. His most pivotal moment occurs when he accidentally enters a material teleporter salvaged from a mysterious unmanned spacecraft. During transport by the Heller Empire, he merges with the kaiju Dostony 1, creating the composite entity Monkey-Dostony.

Despite this physical fusion, Monkey retains his original consciousness and does not act destructively. He displays distress by standing helplessly and crying. Medical analysis by Earth Defense Army doctors confirms the fusion is a mutation irreversible by current Earth technology. Fearing potential loss of control, authorities confine Monkey-Dostony in a cage. Pigu successfully argues for his release on compassionate grounds.

During a later battle against the Heller Empire's Dostony 2, Monkey-Dostony reenters the teleporter and is transported to Titan. This process separates him from Dostony 1, restoring Monkey to his original form. While captured by the Heller Empire on Titan, Monkey resists captivity by scratching soldier Roygar's face. Amid the ensuing chaos, he escapes by reentering the teleporter and safely returns to Earth, reuniting with Pigu and the Science Guard Party.

In the series finale, Monkey accompanies the team to Planet U40 to fight the Heller Empire. Following instructions to avoid targeting, he shelters in Ultria's ruins. When other team members are captured, Monkey initiates a rescue operation after Ultraman Joneus transforms, contributing to their successful liberation. Following the final victory, Monkey remains with the team as they bid farewell to Ultraman Joneus and Amia. Monkey originates from Earth.