Katsuji Matsuoka is a classmate and one of Ryūsei Amanogawa's closest friends. He actively competes in borg battles with his kabuto borg machine Electrical Speedwagon, later enhanced to Electrical Speedwagon V. As a loyal member of the core group relying on Loid's Parts Shop, he consistently supports Ryūsei against opponents and challenges.

Katsuji employs creative strategies alongside Ryūsei and Ken, including disguising themselves as females to confront Night Borger X—a masked battler targeting timid opponents to steal option parts. He also accompanies Ryūsei on rigorous mountain training climbs, where one opponent succumbed to acute mountain sickness from rapid altitude gain.

His family background involves a grandfather who engages in novice borg battles. In one encounter, this grandfather battled Ryūsei using memories as attack power sources. Katsuji himself faced lethal danger when Ryūsei accepted a challenge from a kabutoborg collector but delegated the high-stakes combat to him.