Kagami Yagami is one of the main characters of the anime, and she stands out immediately for her perpetually sleepy demeanor. She is a sixteen-year-old high school girl with a slender build, black hair cut in a precise style, and large red eyes that often appear half-closed due to constant drowsiness. Her signature action is a yawn accompanied by the soft sound "funyaa~," and her manner of speaking often ends with a polite but detached "it is". Despite her languid exterior, she is a caring individual capable of experiencing a full range of human emotions, including joy, sadness, and later, jealousy.

Her true nature is far from ordinary. Kagami is the second oldest of the three Yagami sisters, but she is not a conventional goddess like them. She is a spiritual robot, a divine construct whose body, while appearing human, is entirely mechanical. Her origins lie in a piece of her older sister Tsurugi Yagami's body that was cut off after being cursed by an enemy. This discarded flesh was recovered by a secret organization known as Arahaba-ki, which transformed it into a robotic weapon. Before her sense of self fully developed, she was rescued by Tsurugi, who then raised her as both a sister and a daughter, giving her the family she lacked.

As a being created from a goddess and enhanced with technology, Kagami possesses formidable combat abilities. Her body is an arsenal of hidden weapons, spiritual devices, and missiles. She can manifest mechanical wings from her back to fly and houses the powerful spirit of the Flame Emperor within her, granting her control over intense fire abilities. Despite her immense power, which is said to rival or surpass that of a military force, her divine status is considered lower than that of her sisters Tsurugi and Tama. She also displays very human traits; she enjoys eating, takes naps whenever possible, and is the primary cook for the Yagami household.

The most significant relationship in Kagami's life is with the protagonist, Sasami Tsukuyomi. Their bond began awkwardly, but Kagami became Sasami's first and most important friend after Sasami emerged from being a shut-in. Sasami frequently expresses deep affection for Kagami, even joking about marrying her, and their connection is so strong that their classmates perceive them as being more than just friends. Kagami, in turn, views Sasami as someone incredibly special. Her growth as a person is closely tied to this friendship; she struggles to understand her own emotions, such as when she feels jealous and believes she is "broken" after Sasami befriends another girl, Jou Edogawa. Sasami reassures her that this jealousy is a very human feeling, highlighting Kagami's journey from a near-emotionless doll to a person with a genuine heart. Alongside her close family ties with Tsurugi and her younger sister Tama, her relationship with Sasami is the cornerstone of her development, helping her explore what it truly means to be a friend and to be human.