Ono is a middle-aged man who appears in the everyday scenes of Under Ninja as the next-door neighbor of the protagonist, Kuro Kumogakure. He lives in the same small apartment building as Kuro in Tokyo, and the two rooms are connected through a hole in the closet, so that Kuro can move freely between his own apartment and Ono's. The story never explains why or how this connection exists. Ono is visually unremarkable, with a receding hairline and glasses, and he is presented as an ordinary civilian rather than as a member of any ninja organization.

His background is kept deliberately minimal. Ono lives alone, and his mother occasionally sends him care packages, such as pickled eggplant, items that become regular targets for theft. There is no indication that he has ever trained as a ninja or been involved in the secret conflicts that drive the main plot, and the official character materials classify him among the ordinary people of the story, alongside other civilians who share the protagonist's daily life.

In terms of personality, Ono is reserved and mild-mannered. He rarely complains forcefully, even when Kuro takes advantage of him. His beer is stolen from his refrigerator, his food parcels disappear, and his room is used without permission, yet he tends to accept these intrusions with little resistance. Beneath his unassuming exterior, however, he has some eccentric qualities, including a moment in which he is shown wearing an item of a female neighbor's underclothing that he happened to come across.

His motivations are not deeply explored. He does not have ambitious goals or a hidden agenda. He appears to want nothing more than a quiet, ordinary life, but that goal is constantly disrupted by the strange behavior of the people around him. His main concern within the story is simply coping with these daily annoyances.

Within the narrative, Ono functions primarily as comic relief. His scenes offer a mundane and humorous contrast to the violent clashes between ninja factions, grounding the story in the routines of ordinary apartment life. He also plays a small role in an early plot development: Kuro takes beer from Ono's room and offers it to a neighbor named Kawado, which persuades her to pose as Kuro's mother so that he can enroll in a high school as part of his assignment.

His key relationship is with Kuro, who treats Ono's home as an extension of his own living space. Ono also shares the building with Kawado, another tenant, though direct interaction between them is limited. After Kuro's death, his younger brothers move into Kuro's room, and Ono continues to be subjected to the same kind of treatment, with the new tenants also helping themselves to his belongings. He has no meaningful connection to the ninja organizations or to the central conflict of the series.

In terms of character development, Ono remains largely unchanged throughout the story. He does not gain new responsibilities, skills, or a revealed hidden past, and his role stays consistent as the long-suffering neighbor. Some viewers and readers have speculated that he might secretly be a ninja, but the work itself does not confirm this, and he is never shown using any kind of special ability or participating in combat.

Notable abilities are essentially absent for Ono. He demonstrates no martial skill, no ninjutsu, and no particular talents beyond enduring the intrusions of his neighbors. His most distinctive trait is not a power but a circumstance: the unusual connection between his apartment and the protagonist's, which places him at the center of the story's comedy while keeping him entirely outside its action.