Zushioumaru Sumida is a dog owned by Eia Arakawa, who remains unaware of his origins. He originated as an experimental subject for Dr. Sumida’s Slave Control Method (SCM) device, designed to amplify animal obedience. This experimentation left Zushioumaru permanently fitted with the SCM apparatus.

The dog regularly accompanies Eia, exhibiting distinct protective behavior. During an altercation involving Shiori Adachi and Taiju Nakano, Zushioumaru intervened when Zenichi Bunkyou threatened them. He subdued Zenichi through a bite, forcing submission that functionally registered as an SCM duel loss for Zenichi. This act inadvertently demonstrated Zushioumaru’s capacity to influence human conflicts via SCM mechanics, despite his animal nature.

Zushioumaru occasionally draws attention from other SCM users attempting to exploit his device. Yet he unfailingly returns to Eia, maintaining his role as her companion. His undisclosed background as an SCM test subject positions him as an unwitting link between the device’s inception and its proliferation among humans.