Kento was extremely loyal to The Boss, to the point where he and the other Cross-Eyes created a wax statue of him to worship. After several failed attempts to recreate the research of black powder, his mind became twisted along with his body, becoming a Frankenstein-looking freak like the rest of his companions.
After The Boss' disappearance, he was instructed by the Top Officers to continue his research of Black Powder, knowing they'll soon run out of their primary product to sell. Him, Hiratsuka, and some other members studied the Magic Users' anatomy, using the abandoned cabin of Haru as a hideout, in order to understand how to recreate black powder and also fight them back. They would cut their smoke vessels and destroy the organs that produce smoke to render them defenseless; when they no longer had bodies to test their research on, they started to use their own, losing their sanity in the process.
Prof. Kasukabe, 13, Dr. Vaux, and Jonson went to the Hydra Forest to find Kasukabe's wife, but, to their shock, found Kento and the other disfigured Cross-Eyes in her cabin. Soon after, they attacked the scared humans, but Jonson was able to fight them back and even kill Hiratsuka. In response, Kento commanded one of his men to pursue the humans while he and the other three members took care of the giant cockroach.
Kento and his allies would then engage in a fight with Noi; thanks to their training in hand to hand combat and the study of the Magic User anatomy, they quickly overpowered her and used her body as a bargain to heavily wound Shin when the cleaner came to rescue everybody.
Once Noi woke up and killed the last of his men, Kento, knowing he would be killed soon, took an extremely rare magic smoke he discovered in his research to morph into a grotesque giant. He then chomped down on Noi's upper body in front of an enraged Shin, who used his hammer to carve the Cross-Eyes' face, which released his partner. When the fight escalated to this point, Kento's size allowed him to destroy the whole cabin just by walking in it. Shin, along with the humans, Jonson and the still unconscious Noi escaped, however, before it burned to the ground, caused by multiple candles sparking flammable material. Kento would be caught in the fire, with his last words being, "Boss... Please come back to us... Soon". His body would then be by Tetsujo and Dokuga the next morning after they heard the news about the destruction of one of his last hideouts.