Grey ruled as a powerful monarch over dystopian, technologically advanced Etharia. Born an orphan, he grew up in an institution alongside childhood companions Nico Sever and Cecilia, under Headmaster Wilbeck's care. To sustain the impoverished orphanage, Grey and Nico resorted to burglarizing the wealthy. Their lives shifted dramatically when Grey rescued Cecilia from her uncontrolled ki, forging an unbreakable bond. After the orphanage's destruction and Wilbeck's death, vengeance consumed Grey. He enrolled in a cadet academy to gain power, straining his ties with Nico and Cecilia. He deliberately suppressed his own romantic feelings for Cecilia to prioritize Nico's affection. Months before his ascension to king, Cecilia's kidnapping shattered their friendship; Nico blamed Grey for abandoning the search. Grey later uncovered that his mentor, Vera, belonged to the organization responsible for Cecilia's abduction. He ultimately became king, living a solitary, emotionally detached existence until his mysterious death, initially suspected as assassination or poisoning.

His death was orchestrated by Agrona Vritra, ruler of Alacrya and leader of the Vritra clan. Agrona forcibly extracted Grey's soul due to its unique compatibility with the edict of Fate, a branch of aetheric power. This compatibility arose from Grey's soul structure containing three distinct minds—later manifesting through Arthur representing Spatium (space), Sylvie embodying Aevum (time), and Regis personifying Vivum (existence). Together, these three minds enable comprehension and wielding of Fate, a capability exclusive to Grey across timelines. Agrona intended to reincarnate Grey into a controlled vessel for his strategy against rival Asura leader Kezess Indrath, but Sylvie's intervention redirected the soul into the infant Arthur Leywin in Dicathen.

Reborn as Arthur Leywin, he awoke with all memories and experiences intact. This resulted in exceptional maturity and strategic insight from infancy, though his emotional development lagged behind due to past detachment. As a toddler, he rapidly mastered meditation and mana core formation, displaying combat skills that baffled his parents and their adventurer companions. Early events, including a bandit ambush and separation from his family, tested his resolve. His encounter with Sylvia, a dragon who sacrificed herself to save him, profoundly impacted his worldview; she warned against hatred-driven vengeance. Despite this guidance, his lethal actions against slavers who captured the elf Tessia revealed lingering ruthlessness. Delivering Tessia to the Elven Kingdom of Elenoir, he formed bonds with her family, particularly her grandfather Virion Eralith, who recognized his exceptional swordsmanship and leadership potential.

Throughout adolescence, Arthur grappled with the tension between logical maturity and emotional vulnerability. Protective instincts for loved ones triggered explosive reactions when they faced threats. The continental war between Dicathen and Alacrya intensified these struggles, culminating in Sylvie's physical sacrifice to save his life. This loss nearly shattered his psyche, reverting him to the cold detachment of his Grey persona. His companion Regis and an Alacryan noble named Caera Denoir prevented his descent into madness, helping him redirect his hatred from Alacryans to the true orchestrators of conflict—Agrona Vritra and Kezess Indrath. This shift expanded his purpose beyond personal protection to safeguarding both continents from Asura manipulation.

Post-war experiences, including a year in Alacrya and exploration of the Relictombs, refined his emotional resilience. However, his reliance on the King's Gambit godrune—which suppresses emotions for heightened logic—became addictive. With support from Regis and Sylvie, he learned to balance this ability with his own judgment. His journey revealed his reincarnation was part of a larger cosmic design; as the destined wielder of Fate, he is prophesied to reset the universe's natural order by piercing the "cyst" of corruption. This role intertwines his personal growth with multiversal consequences, framing his existence as a perpetual counterforce against entropy.