De Britto College

Before he became a soldier, he was living with his parents, whom he had been distant to. He would go with them, out of obligation, to listen to Martel's lectures. There, he would meet a young Akane Senri and her caretaker. He would also meet a young Kotori Kanbe, who disliked him due to seeing him pick a fight to protect a young boy who was being bullied in the streets. 

At night, he would search for UMAs at Kazamatsuri. At one time, he went into the forest, where he met the Bayern Knights after encountering a large UMA that he cannot possibly defeat. Kotarou became friends with the leader, Sougen Esaka, who would later recruit him to join Guardian. 

Kotori was also once brought to attend to Martel's meeting, but was managed to convince her parents not to. In exchange, she was to take care of a dog named Pero, who had experienced abuse from his previous owner. 

So Kotarou decided to run away from his home to join Guardian. During the superhuman training, he was paired with Arata Imamiya, Touka Nishikujou, and Nagai, as they were the people who had the lowest scores in the exam training. At first, Kotarou had very little confidence in himself, but it gradually began to grow as he connected with with his other teammates. They all became members by the end, except Nagai, who quit due to her low self-confidence and subsequent loss of powers. 

Their first assignment involved them stationed near an alley inside Kazamatsuri, but Arata and Touka became impatient and headed for the forest instead. Kotarou, concerned, followed them as well. In the forest he witnessed Mikuni, another Guardian, meeting his demise in a fight against the Earth Dragon. 

Kotarou fled from the scene, only to find Akane, who was stuck at the top of a tree. He managed to get her down while being attacked by a hound familiar, but as he tried to escape her, he came across a newborn Key. Just as he was about to attack her, she defended herself with her ribbons, which inflicted on him serious injuries. 

Kotori found him and healed his injuries using her newly-obtained druid powers and a piece of the Key's ribbons, turning him into a half-familair. Due to a head injury, he slipped into coma, aging very slowly. He woke up, having vague memories of the events in the forest. 

In the daisy hill, he found the Key, who asked him to find good memories that would keep the Earth from being engulfed by Salvation. He complied, and while initially having no idea how to find the "good memories" she wanted, the two concluded that ending a major war, which Kotarou figured to be the conflict between Gaia and Guardian, will solve the problem.