Lucile Eris, formerly known as Relx Eris, leads the Eris Swords-Clan, guardians of Roland's royal lineage. He claimed this position by killing his abusive parents to save his younger sister Ferris from their father's assault, an act born of profound protective instinct though he later recognized its monstrous nature.

Severely abused as a child under parents obsessed with combat training, Lucile witnessed their cruelty towards Ferris, whom they branded a "failure." His protective drive intensified with the birth of his sister Iris, fearing his parents might kill Ferris if Iris showed greater potential. At age ten, his father took him underground to confront the sealed "Demon Eris." Overwhelmed by despair that made his eyes bleed, Lucile maintained composure, impressing his father.

During this ritual, Lieral Lieutolu intervened, subduing the demon and implanting the "God Devouring" curse into Lucile to grant power. This curse erased Lucile's memory of their agreement and instilled insatiable hunger. Lieral declared Lucile a "sacrifice" in his plans to aid his own son, Ryner Lute. Over years, Lucile consumed thousands of demons and fragments of the Demon Eris. At eighteen, devouring the demon's core absorbed its memories and emotions, restoring his recollection of Lieral's manipulation. Deep integration made the curse irreversible.

This transformation bestowed the title "The Weaver of All Equations" and immense power. His body became ethereal, immune to physical harm, magical attacks, and "Rule Fragment" artifacts. To ocular abilities like Ryner Lute's Alpha Stigma, he appeared as a "void." His power carried restrictions: he could not leave Roland's borders, and his influence expanded only as the kingdom grew. He gained abilities like telepathy, soul manipulation, and madness inducement through his presence. Signature techniques include "Jade Emperor's Tongue" for power absorption and "The Sword that Cuts Through Light" for nullifying curses.

Lucile orchestrated Sion Astal's rise to kingship, testing his worthiness with Roland's traditional trial of doors inflicting psychological torment. After Sion survived, Lucile guided him to unseal the "Mad Hero" Asruld Roland, binding the ancient warrior to Sion as a weapon. Lucile pledged loyalty but warned he would kill Sion and replace him if he faltered. He operated as Sion's shadow guardian, eliminating threats like the noble Broff Samuels.

Protecting Ferris remained his primary motivation, viewing her as his sole vulnerability. He considered himself and Ryner "monsters" unworthy of her humanity, permitting their companionship only because his "blood-stained hands" could not offer her a normal life. He intervened to save Ferris from assassins, including Miran Froaude, and revived Iris after a near-fatal attack. Despite an aloof demeanor, he occasionally displayed dry humor toward his sisters and Sion.