|“||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, Volume 7, Epilogue
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.
Ost is kindhearted and will sacrifice whatever 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.
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 by it.
- Gravity Reversal・Float: It makes targets float.
- Herculean Strength: Greatly increases strength.
- Soul Light: It replenishes the target's SP.
- Drain Seal: Nullifies SP Drain effects. Effectiveness decreases with repeated casts.
- 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.
- Drifa Diamond Missile: This spell shoots a large diamond spike from Ost's hands.
Ost was active even before the Spirit Tortoise was unsealed and on a rampage. As part of the Spirit Tortoise, a Guardian Beast, she was supposed to be a backup to fight against the waves in case the heroes failed or weren't strong enough. However, Guardian Beasts are considered a last resort since they fulfill their terrible role by sacrificing roughly 2/3rds of living creatures in their world.
Ost's specific role was to "pave the way for the Spirit Tortoise's awakening." She was to manipulate countries in order to have sacrifices ready in case the Spirit Tortoise was to be unsealed and she was to supply hints to the Heroes about how to break the Spirit Tortoise's seal if needed.
After Naofumi receives word about mysterious monsters across Melromarc, Ost appears to Naofumi, pleading for him to kill her before "she" goes on a rampage. However, her appearance is brief and cryptic so Naofumi decides to ignore her pleas.
With the Spirit Tortoise being defeat for the first time, Naofumi starts cleaning the streets of the Spirit Tortoise familiars. However, this time Ost was able to manifest herself and stay permanently. She warns him that the Spirit Tortoise is not actually dead and that it will revive again. As Naofumi receives word about the Guardian Beast's revival, he then introduces Ost to Queen Melromarc and her forces. Ost provides all the information she has on the Spirit Tortoise's body and then supports Naofumi and his party who were trying to slow it down from destroying Melromarc.
When Fitoria arrived to confront the Spirit Tortoise herself, Ost joined the expedition party to travel inside the Spirit Tortoise. Working in tandem with Fitoria, the two groups are able to destroy the heart and the head at the same time which permanently kills the Spirit Tortoise. Naofumi's party, including Ost, then head further into the Spirit Tortoise to see who is responsible for the terrible catastrophe. In the core of the beast they find Kyo who had stolen the energy that the Spirit Tortoise had collected during its rampage. They fight him and are almost overwhelmed. At Ost's final wish, Naofumi used the Spirit Tortoise shield she had unlocked to destroy the Spirit Tortoise's core and defeat Kyo. Kyo, seeing that he was defeated, escaped to his world with the Spirit Tortoise energy he had collected. However, the Heroes rush to Ost who is slowly fading away due to her core being destroyed.
As she lays dying, Ost tasks the heroes with reclaiming the Spirit Tortoise energy that Kyo had stolen and then used her authority as a guardian beast to allow Naofumi to travel between worlds. After a tearful goodbye, she fades away.
After Naofumi was mortally wounded by Takt during his ambush, he was able to commune with the Spirit of the Shield in his near-death state. Within his shield, he found Ost and Atla who were talking on the Shield's behalf. They warn him of the "World Eater" behind the waves and then offer Naofumi the option to bring him back to his original world which he refuses. During their conversation Ost hints that there is a way to bring her back to the world of the living, however she is perfectly happy in the shield. After their conversation, Naofumi wakes up from his unconscious state. Since communing with the spirit of the shield, Naofumi could "communicate" with Ost and Atla by the glowing gem on his shield.
Ost sees Naofumi as the only competent Legendary Hero that had been summon. She sought his help to help destroy "her" body which he eventually accepted. Ost proved both useful and selfless during their time together and she gave Naofumi many gifts including learning about Liberation-class magic and the ability to travel between worlds. Naofumi thoroughly appreciates the sacrifice she went through to save the people of Raphtalia's World and was sad to see her go. He mentions that if he could resurrect people, he would bring Ost back in a heartbeat.
Ost and Mirellia had briefly met each other before the Spirit Tortoise catastrophe during a world conference. At that point, Ost was manipulating one of the kings present so Mirellia initially had a terrible opinion of her. However, as Ost proved herself Mirellia became more cooperative and they were able to work together.
- In the English version of the Light Novel, her hair 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.
- Her first name comes from the German word for east (Where the Spirit Tortoise came from), while Horai is the Japanese name for Mt. Penglai, the mountain carried on the back of the Spirit Tortoise.