Kazuma Tamura is a seven-year-old boy who lives in Moonlight Town with his family, which includes his grandfather Tommy, his mother Ai, and his father Makoto. His mother works part-time at a local supermarket, while his father is a salaryman, making the Tamura household a typical example of a modern Japanese family. Bursting with energy and natural athleticism, Kazuma is an active child who spends much of his free time engaged in physical activities and outdoor play.

His life takes an unexpected turn when he encounters Ojarumaru, a young prince from the Heian era who has been accidentally transported to the present day. Kazuma plays a fundamental role as Ojarumaru's first and most important friend in the modern world, taking on the responsibility of helping the displaced prince integrate into the Tamura home. He becomes Ojarumaru's guide to contemporary life, patiently explaining modern inventions, social customs, daily routines, school life, local festivals, and the functions of common household objects that are entirely unfamiliar to a noble from a thousand years in the past. On many occasions, Ojarumaru is shown hitching a ride on Kazuma's back or his backpack to avoid exerting himself, demonstrating the easy physical familiarity between them.

Kazuma is characterized by his consistent kindness, patience, and a grounding influence that provides a necessary counterbalance to Ojarumaru's more mischievous and whimsical nature. Amidst the supernatural encounters that the pair often find themselves in, such as the recurring pursuits by the Little Oni Trio who are seeking to recover Enma's scepter, Kazuma often serves as a stabilizing figure. He regularly joins Ojarumaru on adventures and misadventures throughout Moonlight Town, accompanying the prince on explorations, interactions with the town's eccentric residents, and playful engagements with other local children. His knowledge and explanations help Ojarumaru navigate both cultural misunderstandings and the emotional challenges that stem from being displaced from his own time and home.

The relationship between Kazuma and Ojarumaru evolves into a steadfast friendship that is characterized by mutual reliance during both mundane activities and truly extraordinary situations, such as encounters with time travel or other figures from the Heian period. While they are bonded like brothers and share a deep connection, they are not immune to ordinary childhood conflicts and will occasionally feud over trifles, such as the coveted pudding made by Kazuma's mother, Ai. The Tamura family as a whole serves as Ojarumaru's primary support system, with Kazuma actively facilitating his friend's adaptation to the family's household rhythms and customs. Kazuma's own ordinary childhood experiences of attending school, sharing family meals, and playing neighborhood games provide a stark but comfortable contrast to Ojarumaru's aristocratic background, forming the heartfelt backdrop for their shared daily life. His primary ability, beyond his physical energy, lies in his role as a bridge between Ojarumaru's historical origins and contemporary Japanese society, using his actions and patient explanations to help his friend survive and thrive in a world that is not his own.