Onikiri is one of the two will-o'-the-wisp child spirits, known as onibi-warashi, who serve at Mikage Shrine alongside a fellow spirit named Kotetsu. In its usual form, Onikiri appears as a small, childlike yokai with a pale complexion, a flat and somewhat rectangular head, and traditional Japanese clothing, and it always wears an okame mask, a comical mask with closed eyes and painted features that hides the spirit's real face. Like other powerful spirits connected to the shrine, Onikiri can shapeshift and take on a more humanlike appearance when interacting with humans requires it.

Onikiri's origins are not explored in detail in the anime, but it is strongly suggested that Onikiri and Kotetsu have served the successive land gods of Mikage Shrine for many generations. This long history explains their deep attachment to the shrine and their insistence on maintaining it properly. When Nanami Momozono becomes the new land god, Onikiri and Kotetsu are among the first to explain her duties, including the task of keeping written records of the prayers of humans.

Onikiri's personality is marked by earnestness, discipline, and a strong respect for rules and order. Emotional expression is restrained, and Onikiri tends to prioritize completing assigned tasks over personal feelings or situational judgment. Onikiri can also display a sharp, sarcastic wit. At the start of the story, Onikiri keeps a cautious, formal distance from Nanami, having learned through long experience that humans who take on divine responsibilities do not always stay.

Onikiri's central motivation is loyalty to the shrine and its god. Its actions are guided by the belief that a shrine must continue to function even when its god is absent, and by a commitment to established routines and faithful service.

In the story, Onikiri works behind the scenes as a caretaker of Mikage Shrine, managing its daily operations, informing Nanami of important events, and offering advice on her divine responsibilities. Along with Kotetsu, Onikiri also provides practical help in the spirit world, such as guiding Nanami past demons and other dangers early in her time as a god.

Onikiri's key relationships include a cooperative partnership with Kotetsu based on a clear division of shrine duties, a master and servant bond with Mikage, the former land god, and a relationship with Nanami that evolves from guarded formality into genuine fondness, loyalty, and dependable support. Onikiri also holds great affection for Tomoe, the fox familiar of the shrine.

Over the course of the series, Onikiri's initial wariness and distrust of Nanami gradually give way to trust and warmth, and Onikiri shows a softer side while helping Nanami through various challenges. A memorable moment in the fourth episode of the second season sees Onikiri temporarily grow much taller when struck by the power of the Lucky Mallet, with the kimono, bow, and mask growing along with the spirit.

As a will-o'-the-wisp child spirit, Onikiri does not possess an ordinary physical body and can appear in a small spirit form, but it can assume a humanoid shape when needed. Onikiri can also shapeshift and change size, and it is able to move between the human world and the spirit world. The spirit holds the knowledge needed to support a land god, from record keeping to assisting in spiritual journeys.