Zen Tachibana and his sister Ai operated as core members of D-Force, a specialized defense unit combating humanity-threatening intelligent dinosaurs. He sustained near-fatal injuries during the catastrophic explosion of a prototype super-tank—an early predecessor to the Izenborg system—that also killed their father. To survive, both siblings underwent extensive cybernetic augmentation, with Zen receiving replacements for his right half.

These modifications enabled synchronization with their advanced combat vehicle. Through the Izenborg fusion process—derived from their names and cybernetic nature (Ai + Zen + Cyborg)—Zen inherited biological human components to become the pilot Izenman. He commanded the converted combat vessel Izenborg-Go against dinosaur forces.

Later in the conflict, Zen and Ai achieved a "second step fusion." In this state, Zen formed the primary physical structure of the giant humanoid Izenbo. His sister Ai, housed within a modified section of the Izenborg-Go embedded in Izenbo’s head, supplied energy and computational support during this combined form.