Ost Horai embodies the humanoid soul of the Spirit Tortoise, a Guardian Beast forged as a final safeguard against the Waves of Catastrophe. Bound to a harrowing duty, she harvests souls through incited wars to fuel the Tortoise’s energy—a process demanding the death of two-thirds of humanity if fully unleashed. Though tormented by the fear of dying reviled and isolated, she clings to the belief that her actions serve a greater purpose. Her appearance belies her grim task: a youthful figure with warm brown hair coiled into a flower-adorned chignon, draped in a green Chinese-style dress patterned after a tortoise shell. She wields earth and gravity magic, conjuring hypergravity fields, harnessing energy from the Tortoise’s familiars, and fortifying allies’ defenses or attacks. Her regenerative powers link inextricably to the Spirit Tortoise’s core, anchoring her existence to its fate. When the Tortoise awakens prematurely under Kyo Ethnina’s control, Ost seeks Naofumi Iwatani’s aid, pleading for the beast’s destruction. Joining his party, she battles Kyo and the rampaging familiars, her cryptic demeanor gradually softening. She comforts Rishia Ivyred and Raphtalia, voicing remorse for the suffering her role inflicted, even as she strategizes against their foes. To halt Kyo’s exploitation of the Tortoise, Ost sacrifices herself, merging her power with Naofumi’s Shield of Wrath in a cataclysmic strike that destroys the core. Mortally wounded, she entrusts him with reclaiming the stolen energy and grants him the ability to traverse worlds, her form dissolving as her task concludes. Her soul endures within Naofumi’s Shield, emerging later as he communes with the Shield Spirit. Though hinting at a path to resurrection, she expresses peace in her current state, offering counsel from her ethereal refuge. Once marked by mutual distrust, her bond with Naofumi solidifies into profound respect—he honors her selflessness, while she admires his unyielding resolve. Ost’s journey mirrors the paradox of her existence: a destroyer forged to protect, a soul burdened by duty yet redeemed through sacrifice. Her legacy echoes the weight of survival in a fractured world, where identity and purpose intertwine amidst ceaseless calamity.