Koyomi Yamanaka (山中 暦 Yamanaka Koyomi) is one of the main characters of SSSS.DYNAZENON. He is a 33 year old NEET who gets caught in the conflict with Dynazenon and the Kaiju, becoming one of the mecha's four pilots, along with Gauma, Yomogi Asanaka and Yume Minami.
Some time in middle school, Koyomi met Inamoto. He made a promise to keep her bad behaviors a secret. At some point, they found a stash of money in an abandoned building. Inamoto asked to go out with him wherever they wished with t hat money, but Koyomi refused and ran away.
In his adulthood Koyomi became a shut-in NEET with a sedentary lifestyle, Koyomi seemed to have little interest in the world around him. This started to change when his cousin, Chise Asukagawa, dragged him along to go see the strange phenomenon in which entire buildings started floating in the sky. Soon afterwards, the Kaiju Shalbandes appeared in the city. When Dynazenon appeared, Koyomi was forcibly taken by the mecha and subsequently became one of its four pilots, shocking the cousin duo.
After defeating Shalbandes, Gauma passed Dynastriker to Koyomi and told him and the others to meet again the next day to begin their training. Koyomi goes under the pretense that he is being "employed" and that he might earn some money. Kaiju Eugenicists made their presence known to the Dynazenon pilots and called out Gauma as a traitor, putting a sense of distrust into the others. Unexpectedly, Inamoto and Koyomi met up again while on the street, and exchanged contact numbers after a brief conversation. When the Kaiju Burnaddon appeared, they were unable to unite their hearts because of their distrust towards Gauma, but were able to do so once again after he told them the real reason he pilots Dynazenon - so that he can meet with the person who entrusted it to him. Inamoto later invited Koyomi out to have a meal some time, making Koyomi reserve some time off training to meet with her.The
When he does get to meet her, they get a bit drunk until Inamoto's husband arrives. Things start to go downhill for Koyomi as his past regrets from his childhood resurfaced, and eventually they parted with each other, with Koyomi going to a Seven 21 shop. There, he met with Mujina, who said to him that they were quite alike as they both didn't know what they truly wanted. Having forgotten his umbrella back at the restaurant, Mujina took the chance to give Koyomi her umbrella while stealing Dynastriker in the process while he was too drunk to notice. Gauma scolded him upon finding out, and used Dynadiver to track down the missing part, encountering the Kaiju Eugenicists in a warehouse. They fight and get Dynastriker back, and Gauma attempted to kill them only for the warehouse to come crashing down on them. Later, they sortied against Bullbind but almost lose. Gridknight suddenly appeared and managed to drive away Bullbind, but the damage to the city was already done. Koyomi found Inamoto's husband, Arata, seemingly dead, and showed some hesitation to help him when he showed signs of life. However, Koyomi remembered that prior to that, he told Inamoto that his job was "helping people", calling emergency rescuers for help. Inamoto gave him a call to thank him, but Koyomi hung up the call midway to have a lollipop with Chise instead.
During Garnix's attack, Koyomi was sent back to the time in school when Inamoto found a bag full of money and asked to go with him somewhere. Instead of running away, he accepted her offer, becoming "lost" in his past. Fortunately, Yomogi managed to hold onto Dynasoldier and kept his awareness. Forcing his way into Kyomi's past, Yomogi helped him regain his awareness of the "present", and brought him back but he received an "S" shaped scar on his ankle in the process. Koyomi helped the others destroy Garnix.
After Garnix's defeat, life seemed to go back to normal as the Kaiju Eugenicists ran out of Kaiju seeds. Koyomi had attempted to resume with his life, but struggled with writing a resume, adding "Dynastriker" as part of it. Just as everything seemed peaceful, Gagula attacked, and Koyomi had to go out to battle once again. Yomogi and the others then went to retrieve their parts of Dynazenon to pilot together for the last time. Even with their teamwork, they were still being overwhelmed by Gagula until Yomogi used Instance Domination on it to force it to move its mouth up and provide the others with an opening to attack. With everyone's efforts, they manage to defeat Gagula and the Kaiju Eugenicists. However, Dynazenon loses its color and Gauma seemingly dies.
Three months later, they visit the bridge where Gauma made his home before Knight and the 2nd leave with Dynarex and Goldburn. Koyomi has gotten a haircut and a suit, and revealed to Chise that he had been going to interviews but failed all of them, only to get a job because Arata helped him get one.