Youji Hiyama, a sixth-grader in Class 6-2 at Harukaze Elementary, comes from an elite family that disapproves of his academic focus and love of reading. This bookish "teacher's pet" harbors a secret crush on his homeroom teacher, Yayoi Yuri. Initially a timid "Well-read Cowardly Lion," he possesses underlying determination. His aptitude for mechanical design makes him vital to the class team; he collaborates on building the Boueigar robot and programs the Go-Saurer using Basic-like code.

His role transforms when he gains control of Grantops. After the Boueigar fails in battle and falls into a crevasse, the ancient protector Eldoran appears, rebuilding it into the powerful Gransaurer and entrusting Youji as its permanent pilot. He subsequently operates Grantops (Triceratops form), Gransaurer (humanoid form), and the Granjet. Within the trio of main pilots, he embodies the Superego, contrasting Kenichi Minezaki's Id and Kinta Shirogane's Ego. His combat style features the "Gran Bomber" attack, throwing an explosive crystal like a boomerang, while Grantops employs a ramming tactic with retracted legs. Gransaurer is essential for forming the ultimate robot, King Go-Saurer, combining with Go-Saurer and Magnasaurer.

Key episodes highlight his development. During Kenichi's ruptured appendix crisis, the usually reserved Youji steps up. In the climactic battle against the Beast Kaiser, with half the class incapacitated by illness, he helps lead King Go-Saurer. After the King Blade shatters, he participates in summoning a replacement weapon and perseveres, contributing to victory. He later shares the shock of discovering Earth's mechanization. A subplot involves his unrequited feelings for Yayoi Yuri occasionally distracting him. Despite his intellectual nature and initial reluctance, piloting Gransaurer forces him to confront danger directly, transforming his bookish caution into actionable courage defending Earth.