Tatsuhiro Someya is a Japanese bodyguard initially employed by Shichisei Hiruma before joining Ichi Security, founded by Ichiro Hiruma. His early background includes single-handedly defeating a yakuza attack. Though he possessed the talent to become a professional football player and received an NFL scout's offer, he declined.

Someya first appears during the Annex I Arc in 2619 A.D., stationed outside a bar where Shichisei Hiruma and Honda Kou converse. Alongside Hyuga, he monitors the situation, discussing operational details like the Annex I mission's shift to "Plan Delta" and the A.E. Virus outbreak affecting individuals such as Sakurato Harukaze. While guarding the location, Russian assailants attack, attempting to abduct Honda. Someya neutralizes them using only his fists, explaining his unbuttoned suit allows quick weapon access but confirming he relies solely on physical combat.

Regarded as one of the strongest characters, Someya possesses exceptional combat capabilities. He defeats Terraformars—humanoid cockroaches known for immense strength, agility, and tough carapace—without undergoing transformation, using minimal effort and countering their high-speed attacks. After transforming, his strength triples. He employs a retractable baton as his primary weapon, using it to defeat a Brown Bear Terraformar while transformed. His combat prowess places him on par with Joseph Newton.

Someya's physical feats include lifting and kicking Terraformars, surprising them with his speed, and resisting attacks from powerful variants like the Bear Terraformar. He demonstrates tactical awareness in combat, preparing to engage multiple opponents and leveraging his environment.