Monmo is a robotic companion designed like a tanuki, functioning as a mechanical motorcycle for interstellar journeys with the protagonist Yuu. As a core member of Yuu's team within the Cosmic Security Company, Monmo assists in investigations and space patrols. This role leads them to Planet Norg after detecting a distress signal; their spacecraft crashes upon arrival, plunging them into the planet's conflict against the demon Morgan.

In the OVA *Cosmic Fantasy: Galaxy Cougar's Trap*, Monmo participates in space missions. Responding to a ship's distress call, Yuu and Monmo board the vessel and confront the space pirate Beluga. Trapped with a proton bomb, Yuu uses his psychic powers to shield them both from the explosion, leaving both Monmo and Yuu unconscious for an extended time.

Monmo later reappears in the sequel *Cosmic Fantasy 2: The Adventures of Van*, continuing as part of Yuu's team alongside Saya and Nyan. The group aids the new protagonist Van in his quest to free Planet Idea from the wizard Galum's tyranny, demonstrating Monmo's ongoing involvement in their interstellar operations.