Ryuzu Myoujin emerges in the Shinemon Arc as a scientist dedicated to uncovering the link between Earth and Cray, the card game's fictional planet. He developed technology to bridge these worlds, collaborating closely with his friend and colleague Rive Shindou. Their joint experiments focused on detecting distinct energy signatures and potential dimensional interactions, including efforts to establish contact with Cray's entities.

A critical experiment involving Rive's young son, Chrono, accidentally forged a connection between the boy and a Gear Chronicle unit named Chrono Dran, confirming Ryuzu's theories on interdimensional links. However, Rive uncovered Ryuzu's underlying plan to capture and exploit units from Cray. Rive sabotaged the experiment, fleeing with Chrono Dran and several blank "Depend Cards" intended for unit capture, subsequently vanishing. This betrayal inflicted deep guilt and trauma upon Ryuzu.

Following Rive's disappearance, Ryuzu abandoned his original ambition of harnessing Cray's power to create a perfect world on Earth. His character shifted markedly, displaying remorse and emotional withdrawal. He continued research at his laboratory, Ryuzu Lab, but shifted focus away from interdimensional manipulation for grand designs. Employing researchers like Yuki Mishima, Ryuzu Lab maintained a connection to Card Capital, particularly through initiatives supporting orphaned children, including Chrono.

Ryuzu's actions indirectly impacted Shinemon Nitta. Shinemon experienced a Heroic BSoD and lost his passion for Vanguard following an incident at Ryuzu's lab involving Rive's loss, contributing to Shinemon's defeat against Kanzaki. Despite stepping back from his earlier goals, the consequences of Ryuzu's past experiments continued to influence events within the arc, shaping characters' understanding of Cray's existence and the dangers of forced interdimensional contact.