Zen Tachibana serves as a core operative in D-Force, an elite unit combating an invasion by intelligent dinosaurs aiming to reclaim Earth. He fights alongside his sister, Ai Tachibana, forming a dedicated sibling duo protecting humanity. During prototype super-tank testing—the predecessor to Izenborg—a catastrophic explosion fatally wounded their father and inflicted near-fatal injuries on Zen and Ai. To survive, both underwent extensive cybernetic reconstruction: Zen’s right half and Ai’s left half were replaced with robotic components, enabling synchronization with their combat vehicle.

This cybernetic link allows Zen and Ai to merge into the unified combat system Izenborg. Triggered by their synchronized call "Ai! Zen! Cross!" paired with a cross gesture from their robotic hands, the fusion sees Zen inherit both human halves to transform into the humanoid pilot Izenman. Simultaneously, their tank Izen-I converts into the aerial vehicle Izenborg-Go, with Ai embedded as its Mother Computer, supplying energy and tactical oversight. This fusion empowers them to engage dinosaur forces effectively.

Midway through the series, Zen undergoes physiological evolution. He achieves a "second-step fusion," merging with Ai and Izenborg-Go to form the colossal humanoid warrior Izenbo. Zen pilots Izenbo’s primary body while Ai—housed within a modified Izenborg-Go unit integrated into Izenbo’s head—provides computational support and energy. This form confronts larger dinosaur threats directly.

Zen’s conflict escalates beyond the initial dinosaur army commanded by the tyrannosaurus Ururu. Later adversaries include the alien entity Dinosaur Satan Gottes and his partner Zobina, manipulating dinosaurs from the Black Nebula. The series concludes with Zen and D-Force expanding operations into space to counter persistent extraterrestrial dangers.

In the documentary *Return of Izenborg*, featuring newly filmed tribute sequences, Zen appears alongside Ai in modernized visuals. The documentary reaffirms his battles against Ururu and other dinosaur commanders, maintaining continuity with the original premise and emphasizing his enduring legacy.