Sizumu is a member of the Kaiju Eugenicists, dedicated to reshaping the world through kaiju. Five thousand years ago, he and his comrades, including Gauma, used their kaiju-controlling powers to bring prosperity to a nation, earning them the title "dragon controllers" from neighbors. This alliance shattered in mutual betrayal. Gauma, shifting loyalty to the kingdom's princess, killed Sizumu and the other Eugenicists during the conflict.

Revived in the modern era, Sizumu and the Kaiju Eugenicists regard Gauma as a traitor for piloting Dynazenon against their kaiju. Sizumu infiltrates Fujiyokidai High School as a transfer student, sharing classes with Yomogi Asanaka and Yume Minami. He deliberately engages them in discussions about kaiju, revealing that kaiju emerge from human desires rather than being Eugenicist creations, and states their goal to create a world where kaiju are "necessary."

Sizumu possesses unique abilities: Instance Domination to control kaiju and induce gigantification, and Kaiju Empathy for precise perception of kaiju thoughts. The kaiju Gagula resides within him, granting powers like emitting repulsive shockwaves and generating defensive barriers. He primarily controls Gagula and previously directed the kaiju Neophobia.

His actions escalate when he stalks Yomogi and Yume, ambushing them to feed on their emotions. After intervening in a confrontation between them and Gauma, Sizumu unleashes Gagula to destroy the city. During the final battle, he confronts Yomogi inside Gagula, questioning why Yomogi rejects the freedom offered by kaiju powers in favor of embracing human vulnerabilities and bonds. Unable to comprehend this perspective, Sizumu departs as Gagula is defeated, leading to the Kaiju Eugenicists' downfall.

Sizumu's whereabouts after the battle remain ambiguous; he was not observed disappearing during prior events, nor was his body present in flashbacks to the ancient conflict.