Kyo Ethnina, known as the Book Hero, originated as a reclusive online gamer in modern Japan. After his game account was compromised and progress lost, he committed suicide by jumping from a window. An otherworldly power reincarnated him into the world of Glass and her allies, a realm plagued by Waves of Calamity. There, he became the wielder of the Vassal Book weapon, though it resisted him due to his inherent darkness and selfish motivations, limiting his control. He possessed stark white hair with a silvery sheen and eyes described as stagnant and rotten, akin to a dead fish, contributing to a dark, gloomy aura. His attire included a long coat, a belt with test tubes across his chest, leather gloves, and shoes, resembling a scientist or alchemist. After physical death, his remnant soul manifested as a skinny man in his thirties, though his voice remained unchanged. Kyo exhibited rampant egomania, considering himself a genius while belittling others as stupid. He viewed the world and its inhabitants as a game and non-player characters for his entertainment, referring to events like the Waves as an "update." Obsessed with power and immortality, he harbored intense terror toward threats like soul eaters. His background included global travels where he ostensibly solved people's troubles with inventions, during which he saved his childhood friend Yomogi. Long before the main events, he established a hidden laboratory within a sealed mansion shrouded in thick fog, accessible only to those he permitted. His primary scheme involved seeking the Spirit Tortoise, one of four guardian beasts capable of containing the Waves. He traveled to Melromarc, prematurely awakened the beast, and took control. He captured the Cardinal Heroes Motoyasu, Itsuki, and Ren, using their holy weapons as an additional power source for the Tortoise. He manipulated its energy to send familiars worldwide and framed the captured heroes as villains behind its rampage. During the battle at the Spirit Tortoise’s heart, he fought Naofumi and allies, employing gravity magic and offensive skills from the Vassal Book. When confronted by Glass, L’Arc, and Therese intent on arresting him, he destroyed the Tortoise’s core, absorbed its energy, and fled to his home dimension. There, he killed Albert, the Vassal Mirror Hero, and stole the Mirror Vassal Weapon. He conducted twisted experiments, transforming Tsugumi and her companions—former members of Trash II’s harem—into human-guardian beast hybrids using Spirit Tortoise energy and dangerous demonic weapons. He utilized stolen Spirit Tortoise energy to create artificial Waves of Calamity, aiming to maximize his power. His abilities included manipulating gravity fields, opening dimensional rifts for teleportation, and unleashing offensive spells from the Vassal Book: "Composition Form One: Bird of Flame," "Libraria" for multi-angled magical bombardment, and "Forbidden Composition: Revelation" from the weapon’s curse series. He could also conjure energy swords, arrows, and multiple firebirds from the book’s pages. His relationships were defined by manipulation and betrayal. He discarded Yomogi as disposable after stealing the Spirit Tortoise’s energy, ignoring her genuine care and attempts to correct his path. Similarly, he exploited Tsugumi’s group for experiments until they rebelled. He feigned friendship with Albert only to sacrifice him for experiments. Rishia Ivyred developed intense hatred toward him, viewing him as unforgivable and her destined enemy; he was easily annoyed by her ideals and interference in thwarting his plans. The final confrontation occurred at his laboratory. Naofumi disabled his gravity spell and used the Spirit Tortoise Shield to reclaim the stolen energy, transferring the Vassal Mirror to Naofumi. Severely weakened and beaten by Naofumi’s attacks, Kyo was killed by Rishia’s finishing blow. His immortality method—transferring his soul to custom homunculi—failed as his soul was destroyed, preventing resurrection.