Kouki was recently crowned as the "Mr of Mr" (Mister of Misters), the best looking male university student in Japan.
Kouki has one younger sibling. His younger brother is called Hajime Haruki (一晴希, they share the same last kanji of their names 一光希). Haruki is about 5 years younger than Kouki. Both of them dance and both of them were in the Avex Academy together. Haruki made it into "dino 2nd" the unit Kouki used to be in along with Takuya and Junya. Both of them appeared together in an Avex Dance Video tie up with their dance gym (Gold's Gym) and the Match brand sports drinks. Kouki says he and his brother don't talk much these days.
Kouki's family lives in a Buddhist temple, so it can be assumed that his father is the head priest or at least a monk. Kouki says it's a small temple (it is undergoing renovations right now [beginning of 2020], so his family is staying at an apartment - 4K size, Kouki said). His family lives in Kanagawa (his school/uni are in Kanagawa as well and so is his brother's). Kouki attends Obirin University (his brother just entered Obirin High School, he used to go to Tsurumaki Junior High in Hadano City, which is close to Yokohama).
Kouki's family also owns a dog, a brown toy poodle called Kohaku.