Theodor wields innate photon control prowess yet begins as timid and aimless. He enlists in ARKS solely because his childhood friend Ulku aspired to join, despite his aversion to violence and fierce opposition to her facing such peril. When Falspawn devastate the ARKS ship Themis, Theodor finds Ulku's identification in a blazing armored vehicle wreck and collapses upon witnessing a body within the flames. Her apparent death sparks a searing metamorphosis: consumed by vengeance, he pledges to exterminate all Falspawn and grow stronger for Ulku's memory.

This oath drives Theodor to accept antagonist Luther's offer. Luther transforms him into a Deuman, amplifying his abilities. Empowered, Theodor aids Luther in obliterating ARKS facilities—including a data-rich lab on ocean planet Wopal—though he occasionally questions Luther's tactics while obeying commands. During a clash aboard Luther's mothership, Theodor assaults Ash and Matoi with the weapon Clarissa under Luther's orders. He spurns Ash's dialogue, swearing loyalty only to Luther, who he believes can resurrect Ulku. Ash reveals Ulku's survival, but Theodor dismisses her as an illusion and attacks. Only when Ulku embraces him, apologizing, does he recognize her reality, shattering emotionally and surrendering.

After Luther's defeat and the Profound Darkness crisis ends, Theodor reconciles with Ulku. They assist ARKS relief efforts, where Ulku reminds him to handle lunch boxes gently. He then joins Ash's mission to save Matoi, engaging in combat against berserk ARKS members and freezing attackers to shield Ash and Afin.