When Yūma still lived in the Neighbor world, he and his father, Yūgo, participated in the Civil War between Calvaria and Spinthir, helping Calvaria as a favor to a friend of his father. One day, Yūgo told Yūma not to participate in the battle because earlier that day they had found the dead bodies of some of their men, and they were some of the most powerful of their side. Yūma's father suspected it was the result of the other side having a Black Trigger, so he wanted to keep Yūma out of danger. Yūma, however, just pouted and didn't really think much of his father's words.