Kagari is a female youkai known as a kamaitachi, a type of sickle-weasel demon, and she is the youngest of three siblings. Her older brothers are named Raishin and Juuro. Despite being the youngest, she is over four hundred years old. When she takes on a human form, she nearly always appears as a beautiful young woman with long black hair and a simple dress, though her true form is that of a large, lithe weasel.

The background of Kagari and her brothers is marked by conflict with humanity. They were driven from their original home when the land was repurposed for a golf course. This event deeply affected her brother Juuro, who became filled with hatred for humans and began to kill them indiscriminately. Unable to stop their rampaging brother themselves and wishing to prevent further bloodshed, Raishin and Kagari sought out the main protagonist, Ushio, to ask for his help. During the ensuing battle, Ushio showed empathy for the suffering of the youkai, an act that moved Juuro. Although Juuro eventually died at peace after being stabbed by Ushio’s Beast Spear, Kagari and Raishin did not hold Ushio responsible for their brother's death. On the contrary, they became his staunch and loyal allies.

Initially, Kagari is wary of other humans, a sentiment shared by her brother Raishin, due to the suffering their family endured. However, she makes an exception for Ushio, whom she respects greatly and addresses with the honorific "Ushio-dono". Her early attitude toward Tora, the powerful youkai bound to Ushio, is one of disapproval and suspicion, viewing him as a monster colluding with humans. When they first meet, she even goes so far as to cut off Tora’s arm after he speaks ill of her brother. This perspective changes after she learns more about Tora’s own history. She later apologizes to him and expresses her respect for him, and her feelings develop further into romantic admiration as the story progresses.

Kagari's role in the narrative is primarily that of a supportive ally and capable fighter. She and Raishin assist Ushio and Tora in several battles, including defending them against other vengeful demons. She also serves a critical function as a healer, possessing the ability to treat wounds inflicted by kamaitachi attacks and even saving Ushio’s life after he is gravely injured by Juuro. Her speed and agility are notable, allowing her to react to and cut flying bullets. In her human form, she can create long, sharp blades from her arms to use in combat. Her character develops from a proud youkai with a deep-seated grudge against humanity to a devoted friend and ally who is willing to fight and die alongside Ushio. Her feelings toward Tora also evolve from open hostility to deep respect and affection.