Berg, Therese, and New-Look Glass are not in the bowels of the Spirit Tortoise to fight the Shield Hero and his party. No, they’re there to deal with one of their own: a fourth Hero from their world who is only causing shit for his own personal gain. Berg proposes a truce and team-up; Naofumi refuses, but says if they don’t want to fight, they can do whatever else they want.
Just like that, they part ways, even though both parties are looking for the Tortoise’s heart. Instead, Naofumi & Co. come upon a dragon hourglass. Ost recalls that it’s used to indicate how many souls the Tortoise has consumed and converted into the power that would create a barrier against the Waves. Berg & Co. seemingly find the real heart first.
Naofumi’s party soon finds what they believe to be the heart, but encounter the fourth Hero from Glass’s world, who clearly matriculated at Generic Anime Bad Guy U. Long, wild gray hair, glasses, a quasi-military uniform, and a devil-may-care attitude…he sucks, and I just want to punch him. But he is the wielder of the Book vassal weapon, and even Ost’s attacks go right through him.
When this guy doesn’t heed Naofumi’s order to release control of the Tortoise and give back the souls he stole, he lets the heart’s defense system kick in, forcing Naofumi to protect everyone with his Meteor Shield. Filo and Raph fan out and attack the heart, but as they do, they cause Ost to cry out in pain, and she eventually passes out.
This whole time inside the body of the Tortoise, Ost has been doing some serious soul-searching. Once her existence was so simple: collect souls with the greater good of protecting the world from Waves. But then why does she empathize and feel with the people whose souls she’s supposedly designed to take?
The answer, it would seem, is that she isn’t really the Tortoise’s familiar. Even with the Tortoise’s head and heart destroyed, she remains, and opens a path to its deepest depths.
The Book Wielder is there, annoyed they’ve made it this far, and reveals that Ost could be more accurately described as the Spirit Tortoise itself. She is its very core, which means if Naofumi & Co. are to succeed, they must kill her…which is exactly what she asked them to do when they first met.