Onija belongs to the Kaiju Eugenicists, wielding power over kaiju. Originating 5000 years ago alongside comrades like Gauma, they used kaiju manipulation to bring fortune to a country, earning the title "dragon controllers" from neighbors. Betrayed by their own nation while simultaneously betraying it in return, conflict erupted. Gauma shifted loyalty to the princess, killing fellow Eugenicists including Onija, leaving Gauma scarred. Revived in the modern era, Onija views Gauma piloting Dynazenon as treason against the Kaiju Eugenicists' goals. He participates in the group's kaiju attacks, such as those involving Shalbandes and Greyjhom, driven primarily by a desire to defeat Gauma for his betrayal, contrasting with Juuga's interest in Dynazenon's human pilots. Physically, Onija stands 180 cm tall, weighs 72 kg, and has crimson eyes. His red hair covers his right eye. He wears the Kaiju Eugenicists' signature colors, his outfit distinguished by three pink stripes on the right sleeve. He sports piercings on his left ear and rings on his left middle finger and right index finger. His key ability is Instance Domination. Activating it requires reciting the technique's name while placing his hand on a kaiju and splitting his middle and fourth fingers apart, enabling control and enlarging the kaiju to giant size. He specializes in commanding upright kaiju with tails, like Burnaddon. His attempt to control Burnaddon against Dynazenon fails when the kaiju is destroyed in space; debris crashes onto Onija but he survives. After Garnix's defeat, Onija temporarily leaves the group to seek other methods of destroying humanity. His misanthropy leads him to attempt stealing a police officer's gun, resulting in arrest and imprisonment. He escapes when Gagula's attack destroys the prison. In the final battle, Onija and the other Kaiju Eugenicists attempt Instance Domination on Gagula, discovering their comrade Sizumu inside. They willingly allow themselves to be devoured by Gagula to empower its second form. They are ultimately destroyed alongside Gagula during its defeat by Gridknight and Dynarex. A recurring pattern sees Onija narrowly escaping death from falling kaiju debris after battles, happening from Burnaddon's defeat until the penultimate episode. He acknowledges this pattern breaks in the finale when he dies, stating he never expected it. His catchphrase following each near-death experience is, "I thought I was dead...".