Denbo Sanjuro is the primary caretaker and guardian for young Prince Ojarumaru Sakanoue. An anthropomorphic firefly originating from the Heian era roughly one thousand years ago, he oversees the prince's well-being and acts as a messenger, facilitating communication between the Heian-era world and modern-day Japan. This role is crucial following Ojarumaru's accidental displacement through time.

He first encounters the prince in the present after witnessing Ojarumaru's fall through the Moon Hole, locating him at the Sakata Apartment complex. Following directives from the prince's parents, he assumes responsibility for Ojarumaru's daily care within the Tamura household in Moonlight Town. Denbo exhibits recurring romantic tendencies, frequently developing affections for various animals, insects, and occasionally inanimate objects; his consistent interspecies romantic interest is Akemi, a parrot.

He particularly enjoys onigiri, establishing it as his signature food preference. This fondness occasionally surfaces within episodic narratives alongside his romantic subplots. Functionally, he often participates in the prince's explorations of modern society, providing practical assistance and comic relief through his reactions to unfamiliar contemporary elements. His presence contributes to resolving minor conflicts arising from the prince's cultural or temporal misunderstandings.

His role remains consistently focused on guardianship, communication between temporal settings, and participation in lighthearted subplots involving his romantic pursuits and culinary preferences across the series' extensive run, television specials, and the theatrical film *The Promised Summer - Ojaru and Semira*.