The 14-year-old protagonist of the series. As a child he was prone to crying frequently and, due to his weak personality, was often picked up by bullies, prompting Yajima and Risako to protect him and look after him, thus becoming best friends over the years. This all changed when he acquired, by death nonetheless, the powers of a Factor, though in different circumstances: in the manga he acquired them as a child while forced through an off-limits area in an industrial compound, while in the anime he was already a middle-schooler, shortcutting through a shrine to come back to school in time. In both instances, his future Machina Linebarrel, literally crash-landed on top of him and, sensing his life ending, revived him and linked with his life force. This endowed Koichi with super-human physical abilities, yet it also made him very arrogant, prideful and overconfident, so much so that he himself became the actual gang leader of those who used to bully him. This strained considerably his relationship with others, especially his childhood friends, prompting Yajima to knock some sense into him. Only grievous events such as Yajima's own death and the humiliating defeats handed to him managed to benefit his maturity, forging a much more serious (albeit still reckless and a bit vain) character.
He pilots the titular Machina of the series, Linebarrel. The machine is regarded as the most powerful and special Machina of all, as its power output far exceeds any other mecha presented in the franchise. It has a bulky appearance, though it's still capable of very fast performances, and its armament include two massive katanas holstered in wrist-mounted sheaths, and a beam-saber device named the Executor housed in a compartment on the tail of the robot, whose sheer power is so great it can generate immense, bright green bursts of energy, kilometers in length. Also, Linebarrel is the only Machina capable to achieve a so-called Overdrive status, which increases dramatically its performance at the cost of putting a heavy strain on the pilot's body. Linebarrel can change into a more intimidating, jet-black form named simply Mode-B, which grants it instant teleportation.