Kouki was born to a loving family in Kawasaki, Japan with two loving parents, an older sister, and a younger brother to follow. Kouki had a fairly peaceful and uneventful upbringing. Really, the most interesting aspect of his life was just how uneventful it was - notably, not once had Kouki ever gotten sick before. Whenever he scraped his knee or broke a bone, it always healed remarkably quickly. Incidents that might have been scary or even traumatic for most other children passed by without incident for Kouki, simply because he was able to heal and move on so fast.

Apart from a few passing remarks by doctors at checkups about his remarkable healing speed, Kouki didn't really think much of it. His life continued to pass by uneventfully - he worked hard in school, got good grades, and was on track to living a completely normal life. He always figured that once he was done with high school he'd end up in college, then get a nice job, maybe settle down after a while, and that was that.

It wasn't until he had gotten scouted by Hope's Peak Academy as the Ultimate Lucky Student that his life truly began to turn around.

Of course, Kouki didn't want to allow an opportunity like this to slip by. Accepting the offer, he bid his family and friends goodbye to begin his new school life. It was here that things began to go downhill for him. While the quality of his schooling at Hope's Peak was exceptional, and Kouki was overjoyed to be taught by the Former Ultimate Teacher of all people, things started to go wrong when it came to his classmates. Namely, the two that took interest in him in very different ways.

First off was Chinatsu. Their classmate Kayo had set the two up together, and they both hit it off amazingly. Chinatsu was charming, sweet, beautiful - really, Kouki felt as if she was way out of his league. With this being his first relationship, he was content to let her take the reigns. Being the pushover that he was, it didn't take long for Chinatsu to dominate and influence every aspect of his life.

Secondly was Haruka. That was where things went from bad to worse.

Haruka took notice of him and his uniquely fast healing capabilities, and asked him if she could operate on him to test those capabilities. Kouki agreed, albeit a little reluctantly, and his fate was thus sealed. As it turned out, he lucked out incredibly hard when he was born, and had ended up with an almost unnaturally healthy body. While Haruka did not hold a personal interest in conducting any further tests, she did pass him along to a group of medical professionals who would be much more interested in his unique physiology. It was through this that Kouki eventually gained his secondary title of Ultimate Test Subject, and became the source of multiple medical breakthroughs.

Overtime, the boy who lived and dreamed of a normal life disappeared, as he settled into a life where his every move was controlled by others. Though he continued to remain kind, was there not a limit to the amount of strain the human mind can take?

Was he even considered "human" anymore, in the eyes of others?

Kouki wholeheartedly loves Chinatsu and trusts her judgement completely. After all, someone as pretty and charming as her chose to be with boring old him, of all people - he can't possibly think his opinion holds a higher value than hers. While there may have been a point in time where they could once be considered equals, that's far from the case now. These days, the mere thought of disagreeing with her is enough to make him feel sick, and upsetting her in general will quickly send him spiralling into complete panic. Down to his core, he both loves and fears her more than anyone else.