Hayase is the 14-year-old protagonist of the series, and pilot of the titular Machina robot, Linebarrel. He was often bullied as a child, and was thus often protected by his two childhood friends. When the accident from three years ago resulted in him receiving superhuman abilities, it eventually turned him into an arrogant and aggressive individual, who disliked being protected. Liked by the entire student body, and viewed as a model student by the teachers, Kouichi secretly hid his more aggressive personality and became the leader of those who used to bully him until Yajima reasons him out of his behavior.
He grows more sensitive to those around him as the series progresses, realizing Yajima's true intentions and committing himself to fulfilling President Ishigami's expectation of him. He also shows his affections towards Kizaki more often.
His new found abilities, as well as his more aggressive nature, is a result of becoming a Factor for Linebarrel. Apparently, he was inadvertently killed by Linebarrel during the accident, and was revived by the robot through the insertion of nanobots into his body, making his life connected to it. Linebarrel excels at speed and power, and its D-S.O.I.L. is more powerful than any other Machina. The robot possesses two katanas which are stored in the arms, and a gun/sword weapon called the Executor stored in the tail. He is later able to use Linebarrel's Mode-B without suffering as much strain on his body as Kizaki does upon its use. Its Mode-B state also grants it the ability to teleport.