Koyomi Yamanaka is a 33-year-old NEET with brown neck-length hair and blue or green eyes. Standing 178 cm tall and weighing 69 kg with blood type A, his name stems from a classical Chinese poem about a mountain hermit losing track of time, mirroring his own isolated existence. Before adulthood, during middle school, he met Inamoto and swore to keep her misbehavior secret. They found a stash of money in an abandoned building, but he rejected her proposal to run away together, fleeing the situation instead—a choice that left enduring regret. As an adult, he embraced a reclusive, sedentary life, largely indifferent to the outside world. His cousin, Chise Asukagawa, intervened by forcing him to witness floating buildings, drawing him unexpectedly into the fight against kaiju. During the attack by the kaiju Shalbandes, he was forcibly taken into the giant robot Dynazenon, becoming one of its four pilots alongside Gauma, Yomogi Asanaka, and Yume Minami. He initially joined battles believing he might get paid, treating it as a job. Early distrust among the pilots toward Gauma, labeled a traitor by Kaiju Eugenicists, caused tension. They reconciled upon learning Gauma's true motivation for piloting. During this time, Koyomi reconnected with the now-married Inamoto, exchanging contact information. He set aside time from training to meet her, resulting in heavy drinking. This emotional encounter stirred past regrets, and while intoxicated, he inadvertently allowed the Kaiju Eugenicist Mujina to steal his Dynastriker control device after she gave him her umbrella. The device was recovered, but his lapse contributed to Dynazenon nearly failing against the kaiju Bullbind. In a later battle against the kaiju Garnix, Koyomi was mentally transported back to his youth when Inamoto proposed running away. In this vision, he accepted her offer, becoming lost in the memory until Yomogi intervened, restoring his awareness of the present. This experience left him with a permanent "S"-shaped scar on his ankle but enabled him to help defeat the kaiju. After the major conflicts ended, he attempted societal reintegration, struggling with practical steps like writing a resume where he listed "Dynastriker" as experience. Three months later, with assistance from Inamoto’s husband, Arata—who helped despite Koyomi's initial hesitation to aid Arata during a kaiju attack—he secured a job. His appearance transformed significantly, featuring a haircut and formal suit, marking a shift from his NEET lifestyle. A trivia note reveals a discrepancy in his birth date: official records list March 12, while his resume stated March 10. He shares a birthday with Mujina.