Mondo Ooya is an ordinary elementary school student with a deep passion for the Monster Collection trading card game and a longstanding crush on his classmate Rokuna Hiiragi. He begins the story as a regular boy at a local public school, but his life changes dramatically when Rokuna's father, Professor Ichiroubei Hiiragi, invents a way to travel to the parallel Mon World. Recruited alongside Rokuna as a Mon Colle Knight, Mondo quickly overcomes his initial shock at the strange new realm and the silly outfit he is given, embracing his role with determination and courage. His primary motivation is to locate the six Mon Mon Items scattered across the elemental realms of Mon World; once combined, these items can unite his world and Mon World, fostering peace and friendship between humans and monsters. He opposes Prince Eccentro, who seeks the same items for domination, and frequently battles him and his underlings. Mondo's personality is adventurous, spunky, and kindhearted, but he also has an impulsive, flirtatious side—especially around attractive girls—which often provokes Rokuna's jealousy. He tends to slip into a heroic, shirtless mode when he encounters a girl he perceives as needing rescue, though his efforts are not always appreciated. Despite these flaws, he is loyal to his friends and genuinely cares for the monsters he meets. A standout relationship is his bond with a pegasus he meets early in his journey; the creature starts out chibi-like and later grows into a full-sized, loyal partner. Mondo also shares a special friendship with dragons, his favorite type of monster. As a Mon Colle Knight, Mondo possesses the unique ability to merge with any monster that permits him by chanting the phrase Adara Kadara Buu. Through this fusion, he shares the monster's senses and suffers its pain but gains its combat abilities, allowing him to assist directly in battles. He merges most often with water-type monsters, though he has also combined with earth, wind, holy, fire, and demon types. His endurance and physical strength make him the powerhouse of the team. Throughout the series, Mondo lives more and more with the Hiiragi family, and his understanding of the Mon World deepens, yet he never loses his simple dream of seeing both worlds live together in harmony.