Daisuke Yajima, Satomi Yajima's younger brother, possesses brown hair and brown eyes. Orphaned in childhood alongside Satomi, he lives with his sister under significant financial strain. Satomi serves as his sole guardian and provider, working multiple part-time jobs to support them and fund Daisuke's education.

Daisuke suffers from a significant illness requiring continuous medical care. While his condition initially improved, it later deteriorated severely due to deliberate poisoning by the Jahana Group. This relapse demanded urgent, costly medical intervention, including surgery, expenses far beyond Satomi's means.

Daisuke's critical health crisis directly compelled Satomi's entry into the Variable Geo tournament. Facing impossible hospital bills with no alternatives, she participated under duress after the Jahana Group offered to cover his treatment in exchange for her cooperation, placing her under their manipulation.

During the OVA storyline, Daisuke experienced an unexpected, severe seizure, escalating the financial and emotional pressure on Satomi. His profound vulnerability and complete dependence on his sister underscore her motivations throughout the narrative. His potential recovery remains intrinsically linked to Satomi's actions and performance within the tournament.

Daisuke's role is passive yet pivotal, functioning as the primary catalyst for Satomi's storyline and her conflict with antagonistic forces.