Zushiōmaru Sumida originated as an experimental subject of Dr. Sumida, creator of the Slave Control Method (SCM) device designed for animal training. Dr. Sumida's assistant, Setagaya, stole the research and repurposed the SCM for human enslavement. After this theft, Dr. Sumida died by suicide; Zushiōmaru witnessed the event and discovered his creator's body. This traumatic experience fueled his resolve to track down Setagaya for revenge.

Carrying an SCM unit concealed in his mouth, Zushiōmaru searched for Setagaya. During his quest, he encountered Eia Arakawa. Unaware of his history or the hidden SCM, she became his owner. Zushiōmaru maintained his canine identity while living with her, concealing both his connection to the SCM's origins and his mission. His path later crossed Yuuga Ohta's SCM activities, resulting in Zushiōmaru's temporary enslavement to Yuuga. This prevented his pursuit of Setagaya and complicated his relationship with the unknowing Eia.

After Eia defeated Yuuga in a duel, she became Yuuga's master and learned Zushiōmaru's past and objectives. This revelation aligned their goals, leading Zushiōmaru to actively join Eia's mission to dismantle the SCM system and liberate the enslaved. His intimate knowledge of the SCM's original purpose and its corruption informed their countermeasures against users.

Zushiōmaru's narrative evolved from a revenge-driven animal into participation in the broader resistance against human enslavement. His unique perspective stems from being both a former experimental subject and a witness to the SCM's misuse. His background as Dr. Sumida's test subject remains intrinsically linked to the core narrative of the SCM's origin and its deviation.