5000 years ago, the Kaiju Eugenicists, with Gauma being one of them, brought fortune to a certain country through the use of their power to control Kaiju. They were known as "dragon controllers" to the neighboring country. The princess of their country became interested in the power, but then the Kaiju Eugenicists were betrayed by the country, while also betraying the country themselves. Gauma, having shifted his loyalty to the princess over his comrades, killed them and then fell over, and seemingly died, gaining the scars on his face in the process.
5000 years later, Gauma and the Kaiju Eugenicists were revived in the modern day. They now consider Gauma to be a traitor for piloting Dynazenon to fight against their Kaiju.
The Kaiju Eugenicists were behind the attacks of Shalbandes and Greyjhom before they revealed their presence to the Dynazenon pilots, and branded Gauma a traitor. While Juuga was interested in how Yomogi Asanaka, Yume Minami and Koyomi Yamanaka were able to pilot Dynazenon despite being seemingly normal humans, Onija simply wanted to defeat Gauma for betraying them. When the chance arrived, Onija used his Instance Domination on Burnaddon, hoping to use it to defeat Dynazenon. His Kaiju failed and was destroyed in space, and a piece of debris came falling back down to Earth, crash landing on Onija but it did not kill him.
After Garnix's defeat, he left the team to find alternative ways to destroy humanity. He was arrested after trying to steal a gun from a police officer. He is imprisoned but the building crumbled during Gagula's attack. Onija and the other three Kaiju Eugenicists attempt to use Instance Domination on it, but realize Sizumu is inside. They allow themselves to be devoured by Gagula for it to power up into its second form. However, they still lose to Gridknight and Dynarex. The Kaiju Eugenicists are destroyed along with Gagula.
- Instance Domination (インスタンス・ドミネーション Insutansu Dominēshon): An ability that puts a Kaiju under his control, and makes it grow to giant size.
- Every Kaiju from Burnaddon to Neophobia has left Onija in danger of getting crushed by a falling Kaiju part, but he wouldn't die. His teammates would leave the scene just as it was about to happen. This gag did not appear again until the penultimate episode, when Gagula's attack broke open the walls to his prison cell.