Kaori Shirasaki, a Japanese high school student transported to Tortus with her classmates, was celebrated alongside childhood friend Shizuku Yaegashi as one of her school's "Two Goddesses" for her exceptional beauty. Her fascination with Hajime Nagumo started in middle school after seeing him defend a stranger from bullies, prompting her to research manga, anime, and eroge to understand him better. This inadvertently fueled bullying against Hajime.

In Tortus, she received the "Healer" job class, mastering healing magic, diagnosis, and group restoration. Assigned to Kouki Amanogawa's hero party, she trained relentlessly after Hajime's fall into the Orcus Labyrinth abyss, convinced he lived. Witnessing Hajime's return during a demon invasion—physically and psychologically transformed—she confessed her love and defected from Kouki's group to join Hajime, despite his warnings about the harsh journey ahead.

Initially Hajime's weakest party member, Kaori battled an inferiority complex but resolved to grow stronger. Fatally stabbed by classmate Daisuke Hiyama during an assault on the Heiligh Kingdom's capital, her soul was preserved by Tio Klarus's magic. Hajime transferred it into the body of Noint, a powerful apostle of the god Ehit. This granted her silver hair, blue eyes (brown in anime), enhanced physical capabilities surpassing human limits, dual greatsword mastery, and evolution magic. She later dyed her hair black and wore dark armor to avoid association with Ehit's forces.

Kaori blends profound kindness and responsibility with intense determination. Naive to others' romantic feelings or hostility toward Hajime, she is generally cheerful and caring but exhibits fierce anger when Hajime is insulted or her friends threatened. Her loyalty to Hajime is absolute; she rejects Kouki's demands to rejoin the hero party and embraces being labeled a "heretic" upon learning of Ehit's manipulation. Party members note her "closet pervert" tendencies, like indirect kissing through shared utensils or wearing seductive outfits for Hajime's attention. Sexually suggestive situations involving him easily fluster her.

Her combat abilities evolved significantly. As a healer, she weaponized support magic creatively, using barriers offensively. In Noint's body, she gained superhuman physical traits, swordsmanship, and high-speed movement via "Godspeed" restoration magic. Accepting her insecurities during the Frost Caverns of Schnee trials allowed full synchronization with the apostle body, matching Noint's power. After Ehit's defeat, her soul returned to her original body, now modified with apostle flesh by Yue and Hajime, enabling voluntary transformation into an apostle state.

Kaori maintains complex relationships within Hajime's party. She rivals Yue for Hajime's affection yet collaborates with her in critical moments. She shares a deep bond with Shizuku, confiding in her and eventually accepting Shizuku's feelings for Hajime. Her friendship with Shea Haulia and Tio Klarus involves lighthearted competition, though she openly labels Tio a pervert. She harbors lasting resentment toward Hiyama for his betrayal. Post-series, she marries Hajime, develops otaku interests through his influence, and resides with him and his other wives.

In spin-off media, Kaori's "yandere" traits are exaggerated, depicting obsessive behaviors like collecting Hajime's belongings, tracing his past, and maintaining a photo-filled shrine room. These portrayals emphasize her single-minded devotion. Her mother's "spirit of anger" manifests as a demonic mask when she is enraged, symbolizing her protective fury.