Keiji Onizuka is a Japanese crew member of the Annex I project, serving under Squad 1 leader Shokichi Komachi. He is a well-built, fit man with distinct red hair and black eyes. Before joining the mission to Mars, Onizuka led an isolated childhood, spending most of his time at home watching movies from around the world with his mother on an island. This lonely upbringing contributed to his taciturn nature. He channeled his solitude into disciplined training and became a professional boxer, eventually rising to become the lightweight world champion. As a southpaw fighter, his style was that of a counter-puncher who relied on evasion and stamina rather than overwhelming aggression. His promising boxing career was abruptly ended by symptoms of retinal detachment. Although a difficult surgery successfully cured the detachment, Onizuka incurred significant debt in the process of regaining his boxing license. He initially refused to join the Annex Project to resolve his financial troubles, but after his mother's death, he reconsidered and signed on, driven by a sense of romanticism and a personal quest to obtain the best eyes through the M.O. Operation.

In terms of personality, Onizuka is defined by a deep-seated desire to protect his fellow crew members and humanity as a whole. This protective instinct extends even to his enemies, as demonstrated when he once risked his own safety to save a foe, Hong, from a Terraformar. He adheres to a strict personal code of chivalry, refusing to strike women under any circumstances. He upheld this principle even during a brutal confrontation with Xi Chun-Li, who severely damaged his ribs and one of his testicles. Compared to more licentious peers like Marcos E. Garcia and Akari Hizamaru, Onizuka is notably prudish around women, becoming visibly uncomfortable with situations involving nudity or intimate contact, such as the prospect of drinking from Peggy Fortie's stomach or seeing a nude Xi Chun-Li.

During the Annex I mission on Mars, Onizuka played a crucial role in the combat against the Terraformars. His most notable battle was against a Pachyrhynchus Infernalis Terraformar, an insect with an exceptionally hard exoskeleton. Despite suffering severe injuries, including damage to his right eye and left arm, he drew upon his resolve to return to Earth and show his late mother that he could live a normal life with friends and a lover. Fueled by this determination, he delivered a decisive, thunderous blow that defeated the powerful creature. Later, during the events on Earth following the mission, Onizuka continued to fight. When Squad 4 betrayed the project, he infiltrated the Annex 1 base to disable a signal jammer, leading to a confrontation with Xi Chun-Li. Their fight eventually resulted in an uneasy alliance to combat a common enemy, a Honey Badger Terraformar. His combat experience also forced him to face a unique adversary: a Terraformar spawned from his own severed arm, which possessed all the same abilities as him.

Onizuka received his M.O. Operation from the peacock mantis shrimp, a creature that complements his existing skills perfectly. This grants him several remarkable abilities. His punching power becomes incredibly strong and durable, capable of breaking the hardest insect exoskeletons. He also possesses the most advanced eyes in the animal kingdom, allowing him to see clearly in pitch darkness, detect light from the near-infrared to ultraviolet spectrum, and see through camouflage that renders others invisible to the human eye. Furthermore, he has the ability to regenerate lost limbs within a few minutes with little effort. Onizuka also utilizes carapace-integrated power wrists named Gonna Fly Now. These function as both protective armor and weights that restrict his speed. By removing them, he sacrifices some defense for a tremendous and crucial increase in speed and striking power. His personal history and fighting style are explored further in the spin-off manga, Terra Formars Gaiden: Keiji Onizuka.