Izumi Kagura, a third-year student at Oumidai Academy, leads the Astronomy Club while bearing the legacy of the Firefolk—a lineage defined by generations of conflict against supernatural Light Hunters and marked by distinctive red eyes. Though gentle in demeanor, she commands decisively during combat, wielding the massive scythe World Splitter and channeling her supernatural healing ability, "Mend." Her leadership is tempered by persistent self-doubt over her choices.

She first crosses paths with protagonist Ryou Hamura during a Light Hunter attack, rescuing him and recruiting him into the club's nightly battles under the True Moon's blue glow. Her responsibilities include coordinating these defenses while navigating a fractured relationship with her brother, Souji Kagura. Their long-standing theological rift intensifies when Souji allies with the Light Hunters.

In a critical act of protection, Izumi erases Ryou and fellow club members’ memories of their battles, inadvertently reshaping personal dynamics—such as sparking a romance between Ryou and Kyouka. When Souji kidnaps Manami Miwasaka to exploit her Kotodama powers for summoning the Queen of the Night, Izumi confronts him directly. Later, her solo assault on the Light Hunters' tower ends in near defeat until the Astronomy Club intervenes. The siblings’ ideological clash culminates in a final battle atop the tower.

Her bond with the club hinges on fierce loyalty and mutual reliance. Despite her memory alterations, she inspires their return to action, and they rescue her from her suicide mission. Her arc embodies the struggle between her protective instincts as a leader and the isolating weight of her family’s legacy.