Ost Horai serves as the humanoid manifestation and soul of the Spirit Tortoise, one of the Four Guardian Beasts defending the world against the Waves of Calamity. Tasked with harvesting souls to power the Spirit Tortoise's last-resort barrier—a process demanding the sacrifice of roughly two-thirds of all living creatures—she resorted to inciting wars between nations or deploying the Spirit Tortoise's familiars to accelerate collection. Though fulfilling her designated role, she bore significant internal turmoil over being perceived as a villain and feared dying hated and alone. She possesses shiny brown hair styled in a flower-adorned chignon and sharp, Chinese-inspired eyes. Ost wears a green Chinese-style dress featuring a hexagonal turtle-shell pattern, a loose shawl fastened at the waist with lotus flowers, and displays a vivacious, healthy complexion. Her personality blends strategic manipulation with profound kindness. While capable of cruel acts like seducing rulers to provoke conflicts to achieve her mission, this behavior was an act divorced from her true nature. Freed from her duties after the Spirit Tortoise's corruption by Kyo Ethnina, she revealed selflessness, prioritizing others' lives over her own and expressing a longing for a normal human existence. Her greatest joy stemmed from acceptance by Naofumi Iwatani's party, granting her genuine companionship. Ost's abilities focus on earth and support magic, including forgotten human spells. She manipulates gravity fields offensively and defensively using spells like Gravity Field・Extreme Gravity and Gravity Reversal・Float. As the Spirit Tortoise's soul, she could immobilize its familiars and absorb their energy. Her healing and power intensified within the Spirit Tortoise's body, though Kyo's usurpation prevented her from controlling its main form. Cooperative magic included enhancing Naofumi's shields and jointly casting the strengthened support spell All Liberation Aura. Key spells involved Herculean Strength for physical augmentation, Soul Light for SP replenishment, and Black Dragon Flame—synergizing with Naofumi's Shield of Wrath but inflicting burns upon her. Her chronology begins with a cryptic plea to Naofumi in Volume 6 to kill her before the Spirit Tortoise rampages. Following its initial defeat in Volume 7, she permanently manifested to warn of its revival, aiding Naofumi and Queen Mirellia's forces with tactical intelligence. She joined the expedition inside the Spirit Tortoise alongside the filolial queen Fitoria, coordinating simultaneous attacks on its heart and head to ensure permanent destruction. During the core confrontation with Kyo, she unlocked the Spirit Tortoise Heart Shield for Naofumi, enabling Kyo's defeat and the core's destruction. This act fatally damaged her; before fading, she authorized Naofumi's interworld travel to reclaim stolen energy and expressed contentment at dying among mourning allies. In Volume 16, Ost's spirit appeared within Naofumi's near-death Shield, warning of the "World Eater" and hinting at possible resurrection while stating contentment remaining within the Shield. Her consciousness continued communicating with Naofumi through the Shield's gem. She viewed Naofumi as the only competent Legendary Hero, trusting him with her life and mission, and gifting him knowledge of Liberation-class magic and interworld travel. Queen Mirellia, familiar with Ost's manipulative role from a prior World Conference, initially distrusted her but later collaborated based on their shared goal of world preservation.