Ost Horai

"No. I was fated to destroy life, to be hated and scorned. People were to cheer when I died, and yet here you are, sad to see me go. I cannot help but feel joyful."

- Ost

Ost (オスト) is the soul of the Spirit Tortoise, the manifestation of the Spirit Tortoise itself in human form.

Appearance
Ost has shiny brown hair tied into a chignon and sharp Chinese-looking eyes. Her skin is healthy and bright and according to Naofumi, she looks vivacious.

Story
She comes to Naofumi, pleading for him to defeat the tortoise before it goes on a rampage. Naofumi at first doesn't believe her, claiming that the tortoise is dead and that he had defeated it earlier. However, he is proven wrong, and the two rush back into combat. Naofumi heads out to kill the Spirit Tortoise, knowing Ost will die as well.

Personality
Ost is kindhearted and will sacrifice what is needed to protect her friends. She cares more about the lives of others than her own. She wishes that she could have lived a life as a normal human and spent more time with Naofumi as an ally, but doesn't let this desire get in the way of defeating Kyo and ending her own life.

It is noted that her first method of calming the waves is to use her beauty to bring about war and harvest the souls to use them to keep the world safe. Had that not been successful, she would release the Spirit tortoise familiars to speed up the process, only opting to bring about the Spirit Tortoise to forcibly gather the souls required if that failed. Because of this it can be assumed she is capable of manipulation and cruelty, Melromarc's Queen is quite familiar with this side of her. However this is stated not to be her real personality and it is only used as a means to protect the world, as such she doesn't display any of these traits in front of Naofumi's group because by then her purpose had been usurped by Kyo and she had no reason to.

Her greatest regret and fear is that she was meant to die hated and alone because of her mission, which requires her to be a villain, as such she is happy that she got to meet Naofumi's group and to be her true self while with them even if the cause was decidedly negative.

Abilities
Ost mainly uses earth and support magic, along with magic long ago forgotten by humans. She can also manipulate gravity fields around herself, allowing her to cast gravity magic as an extension of said ability. As the soul of the Spirit Tortoise itself, she can halt the movements from other Spirit Tortoise familiars and take energy from them to fuel her own spells. She also grows in power the deeper she's located inside the Spirit Tortoise, allowing her to recover from wounds as fast as the Spirit Tortoise does. However she cannot control the main body of the Spirit Tortoise, due to it having been taken over.

Magic
「Soul Light」Replenishes the target's SP

「Herculean Strength」Greatly increases strength

「Drain Seal」Nullifies Drain effects. Effectiveness decreases with repeated casts.

「Gravity Reversal・Float」Makes targets float.

「Gravity Field・ Extreme Gravity」Produces a gravity field that sucks enemies in it towards the ground

「Black Dragon Flame」Shoots a blazing inferno from ost's hands that burns enemies. The flames have the same properties that Dark burning curse has. it's powered by the shield of wrath, extending the time naofumi can use it by using up the rage inside it. However it also damages ost, covering her hands in horrible burns.

Trivia

 * In the English version of the light novel, her hair ones is described as having a red color. That is likely due to the case that the term 紅茶色 was used for her hair color, which can be translated as black tea color. But because 紅 also means red, the hair was translated as red colored.