Kirie Sakurame is a core member of the immortal organization UQ Holder, holding the rank of Number 9 and serving as the primary financial sponsor for all of its activities. She is a young girl of short stature with light-colored hair, often styled with a braid on the left side of her face and another at the back of her head. Her typical outfit resembles a school uniform, featuring a patterned cap, a dark cape-like top, a skirt, dark boots, and glasses. Due to consuming the Elixir of Eternal Youth, her physical aging halted around the age of thirteen, though her mental age reflects significantly more lived experience. Later in the story, after she loses her immortality, her body ages naturally into a teenager and eventually into an elderly woman.

Initially, Kirie presents herself as extremely shy and reclusive, often avoiding social interaction and hiding behind allies like Ikkuu Ameya. This behavior masks her true personality, which is domineering, rude, and strategically calculating. She frequently displays arrogance and condescension, particularly toward the protagonist Touta Konoe, whom she initially dismisses as incompetent. Despite this harsh exterior, she gradually develops deep romantic feelings for him, becoming fiercely protective and emotionally dependent. A profound fear of abandonment, stemming from severe childhood neglect, underpins her dread of isolation. She first activated her time-reset ability after starving to death at the age of four due to being abandoned by her parents.

Kirie possesses a unique form of immortality centered on time manipulation. Her primary ability is called Reset & Restart, which allows her to create save points in time. Upon death, she automatically reverts to the last save point, retaining all memories of previous loops. This forces her to meticulously memorize events to avoid timeline inconsistencies. Variations of this power include Time Freeze, an ability that pauses time and is activated and deactivated by kissing Touta, as well as Short Retry Action, a limited rewind used to avoid immediate fatal outcomes. After training under the powerful witch Dana, Kirie masters various combat and utility magics, including Accelerate for speed enhancement, Familiar for scouting, Telescopy for long-range vision, Magical Burst for propulsion, and a physical takedown maneuver called Sakurame Frankensteiner. She also forms a Pactio contract with Touta, which grants her the magical card Viator Temporis.

Kirie's role in the story is that of a strategic lynchpin. Her ability to reset time allows the group to gather intelligence, avoid catastrophic outcomes, and tackle enemies far beyond their normal power level. Her relationship with Touta evolves from hostility to a deep romantic attachment. Their first accidental kiss triggers her Time Freeze ability, during which she confides in him about her fear of solitude, and he vows never to abandon her. She repeatedly confesses her love to him, and during battles, she prioritizes his safety above all else. After she loses both of her immortalities to power a spell that returns the group from a parallel dimension, she begins to age naturally. Touta accepts her as his lover, and they eventually consummate their relationship. Decades later, as an elderly woman, she remains devoted to him and later agrees to a polyamorous arrangement with Touta, Kuroumaru Tokisaka, and Karin Yuuki. Throughout the series, Kirie demonstrates significant emotional growth, moving from a reclusive, fearful girl to a courageous and devoted partner, all while her strategic value proves crucial in the final battle against the powerful Mage of the Beginning.