Ko-Oni manifests as a minor yōkai character across distinct narratives, consistently depicted with small stature, gray hair except for a bald crown, oni horns, and potentially traditional attire like a tiger pelt loincloth.

In one setting approximately 400 years ago, Ko-Oni serves the Nura Clan. His hair, once black, has grayed. He associates with yōkai like Nattō Kozō, Tōfu Kozō, and Sannokuchi, enjoys sake, and has a history of playful interactions, including games of tag with the young heir Rikuo.

Another narrative identifies this figure as Shōki, placing him within broader yōkai events. This version participates in the Great Yōkai Sports Festival on Akamata's team and appears in citywide parades influenced by Yato-no-Kami. Historical legend recounts Shōki tormenting ruler Hōjō Tokimasa nightly, evading prayers and repellents. A spiritual vision directs Tokimasa to cleanse his sword. The cleansed sword then autonomously strikes a brazier leg preemptively marked with Shōki's image, delivering a decisive blow when Shōki appears and ending the torment permanently.

Ko-Oni functions primarily as a background or supporting figure without explicit developmental arcs across extended media. General oni folklore tropes—superhuman strength, potential magic, cultural ties to misfortune—may implicitly apply but remain unspecified for this character.