Akari Taiyō is a twelve-year-old girl born on August 1, 2004. She inherited the power of the Sun tarot card from her mother, Taiyou Hinata, a former wielder and member of Sefiro Fiore. Her mother deliberately withheld training to give Akari a normal life, though Akari aspired to become a fortune teller like her. Following her mother's death, Akari lived with her cousin, Fuyuna Shinzaki. Fuyuna succumbed to jealousy, transforming into a Daemonia—a monster born from human despair. During this incident, Akari's powers awakened, resulting in Fuyuna's accidental death and erasure from existence. Light novel material later reveals Akari's father, Hajime Takatori, was a Daemonia researcher for Sefiro Fiore who had been possessed by a Daemonia, explaining Akari's unique ability to hear Daemonia thoughts.

Akari possesses an upbeat, kind, and excitable personality. She easily forms friendships but is sometimes perceived as meddlesome due to her insistence on understanding others' struggles. Her defining trait is her resolve. Initially opposed to killing Daemonia after hearing their suffering voices, she accepts this duty to end their torment, even offering herself to enemies to protect friends. This resolve stems from her trauma over Fuyuna's death and her inability to hear Fuyuna's final words. Her Sun card manifests as a flaming rapier in combat, accelerates plant growth, and later enables her to transmit Daemonia thoughts to others through touch. Physically, she has short orange hair with a crescent-shaped side ponytail, red eyes, and fair skin. Her magical form features longer hair and the flame sword.

Her key relationships drive development arcs. With Luna Tsukuyomi (Moon card), Akari shares a deep bond, trusting Luna for advice. She endures physical attacks to protect Luna when Luna is manipulated into a demonic wolf hybrid, later brokering a deal with the antagonist Cerebrum to restore Luna's humanity. Initially, Seira Hoshikawa (Star card) clashes with Akari over opposing views on Daemonia—Seira advocates ruthless elimination due to a friend's death, while Akari emphasizes understanding. They reconcile after collaborating to help Seira communicate with her deceased friend, developing mutual respect. Ginka Shirokane (Temperance card) bonds with Akari during a crisis involving Ginka's Daemonia-transformed mentor; Akari uses her ability to convey his final words of pride and love, solidifying their trust. Ginka later vanishes after defeating her Daemonia counterpart but returns upon hearing Akari's voice.

Akari's character evolves through relentless trauma. After Fuyuna's death, she joins Sefiro Fiore. She initially refuses to kill Daemonia but gradually accepts the necessity to end their suffering. Her breaking point occurs when Cerebrum exploits Luna's vulnerability, forcing Akari to sacrifice her own safety in a pact to save her. This culminates in Cerebrum trapping Akari in a time loop, forcing her to relive Fuyuna's death repeatedly. Surviving this ordeal strengthens her resolve and highlights her willingness to endure personal agony for others' sake. Her heritage as a half-Daemonia hybrid—revealed when Cerebrum confirms her father's possession—adds layers to her empathy for Daemonia and fuels her determination to break cycles of despair. Light novel prequels further contextualize her mother's history within Sefiro Fiore, underscoring the legacy Akari carries.