on-chan is the central character of the anime built around the mascot of Hokkaido Television Broadcasting, and he is presented in the stories as a boy from the Parade Planet, a world located far across the galaxy. His body is shaped like a flattened yellow sphere with simple limbs and a mouth, and his eyes are formed by the letter o on the right and the letter n on the left, which gives both his face and his name their distinctive spelling. He is roughly the size of a basketball and sometimes wears a small hat, all of which gives him a round, friendly appearance.

In terms of background, on-chan is a Parade Alien who comes from the Parade Planet, and the animated stories treat him as a young, curious character who has come to Earth with the wish to meet many new friends. He is described as a boy living on the Parade Planet, and his everyday life is filled with the search for exciting things. He approaches nearly everything with a positive and forward-looking attitude, and although he is easygoing, self-paced, and somewhat absent-minded, with a clumsy side, his natural optimism quickly lifts his spirits whenever things go wrong.

on-chan is extremely fond of food and eats a great deal, with cheese as his favorite, and after arriving in Hokkaido he develops a strong liking for corn as well. Fishing, insects, and watching television are among his favorite pastimes. In his speech he has a habit of replacing the o sound in Japanese words with on, so greetings such as good evening and good morning come out in his own playful style, a trait that is carried into the animated dialogues and gives him a distinctive way of talking.

His main motivation is friendship: he wants to go to Earth and make as many friends as possible, and his curious nature pushes him to explore his surroundings and look for exciting experiences. Within the story he serves as the protagonist of a series of short adventures set largely in Sapporo and Hokkaido, where he encounters various situations and meets a cast of companions and local characters. In the longer animated adventure, a transfer device developed by the professor sends on-chan and his friends to Hokkaido after a prank by no-chan, and there they help a Tokyo family that is relocating to Sapporo. The group travels with the family and guides them through many obstacles, and above all on-chan helps the family's son Kenta come to terms with leaving his old life behind.

The people closest to on-chan include ok-chan, his childhood friend and girlfriend, who is bright, fond of fashion, and dependable, and who often acts as an older-sister figure for him; guchi, his pet, a strange creature that can change its size freely and is always by his side as a conversation partner; and no-chan, a character who sees on-chan as his rival, prefers opposite things such as bitter and spicy flavors, and behaves in a mean-spirited way while still being difficult to truly dislike. no-chan's pet catagon also appears among the group. In the human world, on-chan bonds especially with Kenta, a fourth-grade boy who loves soccer and video games and initially resists moving from Tokyo to Sapporo; through on-chan's influence, Kenta gradually opens up to his new home.

on-chan's development is gentle rather than dramatic. He remains throughout a positive, food-loving character, but his experiences on Earth, and especially his time with the relocating family, bring out his role as someone who quietly changes the mood of those around him. His most notable ability is his dream power, a force he does not realize he possesses: without being aware of it, he influences the people near him and encourages them through difficult moments, which is exactly what helps Kenta accept his new life. This unconscious power, together with his endless appetite, his curiosity, and his habit of turning words into on-filled expressions, defines him as a cheerful and reassuring presence at the center of the anime.