Eugeo is the deuteragonist of the first half of the Alicization Arc. Eugeo was a child born in a remote village in the Human Empire of Underworld, where he was assigned the Sacred Task of felling a giant tree with his childhood friend Kirito. After a mishap during an adventure to the mountains resulted in his other childhood friend Alice Zuberg being apprehended by an Integrity Knight for breaking the Taboo Index, while his memories of Kirito were sealed by the administrators of the world, Eugeo spent the following six years focusing on his work to avoid thinking about his lost friend, until Kirito's return to Underworld led to Eugeo building the resolve to embark on a journey to become an Integrity Knight himself to rescue Alice, as well as acquiring the skills to wield the Blue Rose Sword he had retrieved from the mountains where Alice had broken the Taboo Index.

Spoiler

One year after departing from Rulid Village, Kirito and Eugeo enrolled at the Sword Mastery Academy where Eugeo broke the Taboo Index protecting his valet and her friend from being sexually assaulted by two high-ranked nobles. Kirito and Eugeo were then arrested and brought to prison at the Central Cathedral of the Axiom Church by Integrity Knight Alice Synthesis Thirty, Eugeo's former childhood friend, who no longer had any recollections of her past. After escaping the prison and learning the truth of the world from Cardinal, the duo fought their way to the top of the Cathedral to acquire Alice's lost memories and defeat the ruler of the Axiom Church. During the final battle against Quinella, the Supreme Priest of the church, Eugeo merged with Alice's lost memory fragment and his sword in order to defeat Quinella's Sword Golem, but suffered fatal injuries when he attacked Quinella after defeating the golem, passing away in the aftermath of the battle.

Spoiler

In *Alicization - War of Underworld*, Eugeo helps Kirito repair his Fluctlight using his own, and the two fight together against PoH and Dark God Vecta using the fraction of his Fluctlight that was left in the Blue Rose Sword.