Kazuma, an eight-year-old boy living in Moonlight Town, attends second grade at Gekkō Elementary School. He resides with his parents, Ai and Makoto, in the Sakata Apartment building. His grandfather, Tomio, lives nearby in a log cabin. Kazuma discovers a young Heian-era prince who falls onto their apartment roof through the Moon Hole. Together with Tomio, he convinces his parents to let the prince join their household.

A brother-like bond forms between Kazuma and the prince; Kazuma often carries him on his back or in his backpack during their explorations. He shows athleticism through soccer and pursues a hobby of collecting rocks, storing them in his room. While generally courageous, he harbors a fear of dogs.

Within his family, Kazuma interacts with his mother, Ai, frequently seen managing household duties in her apron, and his father, Makoto, a soft-spoken salaryman who enjoys beer. His grandmother, Sayuri, is deceased, appearing only in flashbacks or photos. Extended family includes Makoto's aunt, Jun Koshifubuki, and a cousin named Alisa.

At school, Kazuma's closest companions are Kintarō Sakata, a wealthy boy living atop their apartment who favors lollipops, and Komachi Ono, a fashion-conscious girl often checking her reflection with a hand mirror. Kazuma actively participates in their daily adventures and misadventures around town.

His experiences extend beyond the present when he accompanies the prince through the Moonlit Night Pond, facilitating travel between modern Japan and the Heian era. These journeys involve encounters with the prince's fiancée, Princess Okame, and helping evade the pursuing Oni Child Trio seeking a stolen scepter. Kazuma consistently acts as a supportive friend and stabilizing presence amid the prince's escapades.