Katsu Kobayakawa is a fifth-grade student at Goha 7th Elementary, firmly embedded within Yuga Ohdo's main circle of friends. He consistently supports Yuga's endeavors and participates alongside classmates like Ricky Endo in school activities and significant Rush Duel Club events. Katsu joins the group search for a "dueling kaiju" in season 1, collaborating with Yuga's friends and members of the Dueling & Dinosaur Research Club from Goha 2nd Elementary. During this quest, he observes internal group conflicts, such as Professor Diggs prioritizing personal gain over teamwork, though Katsu does not drive the resolution. His involvement consistently reflects group participation and backing of the protagonist's initiatives, rather than individual storylines or distinct personal development.