Born in late springtime of 1988 at the exotic west seaside of Kyushu, Koji grew up as the eldest child and only boy with two younger sisters. He describes himself as "a bumpkin who loves sports and music!" And indeed, he is a pretty good soccer player and a talented dancer. His childhood dream of being a vet changed with the end of his junior high days, when at fifteen years old he saw the TV series Orange Days, in which Satoshi Tsumabuki played the leading part. The play of the also Fukuoka-born Tsumabuki in the praised drama inspired Koji's desire to become an actor. His parents decided to support him and enrolled their son in the local Acting School Himawari in Fukuoka City. Thus, 7 years of soccer training were replaced by drama, dance and music classes, alongside his regular senior high studies.
After one year was Koji casted in mid-2005 as Makoto Kanno in the comedy TV series Rocket Boys as the snobbish and "holier-than-thou" main antagonist of later D-BOYS fellow members Endou Yuuya and Yanagi Kotarou. Although the series was filmed earlier, it would take over six months until its broadcast in early 2006. Meanwhile, after the filming ended, Koji was surprised by his parents with a letter that stated the teenager made it to the final round of Watanabe Entertainment´s 2nd official D-BOYS audition. With Fukuyama Masaharu´s song "Himawari", he won the Grand Prix 2nd Winner Award on July 31, 2005 in Tokyo. This new, junior member made his debut shortly afterwards in December at the 3rd D-Live Event - an entertainment stage show featuring only D-BOYS.
Seventeen years old in the spring of 2006, Koji finally moved after constant commuting from his rural and idyllic hometown in Fukuoka to live by himself in the over 1,100 km distant and turbulent Tokyo - having to leave family and friends behind. "Despite seeming sad, they said 'you're amazing'. Because everyone was like that, they gave me a really big farewell party in my honor. Everyone was there, my family and all my friends." The parting words from his father were meant to encourage him: "You can blossom!". But the new, different environment and sudden loneliness distressed the young man. Only the acting could make him forget the pain of separation, even just temporarily. "I became lonely and homesick. I cried after talking with my family on the phone and I cried when I looked at pictures of my friends and family. I think I cried for about two months, possibly." But work wasn´t lacking. All D-BOYS starred together in their first own TV production "DD-BOYS". The series was half drama and half adventure game with different challenges for the boys. They played pretty much themselves, and so was Koji the still timid bumpkin who was trying to spread his wings, making new friends along the road and learning how to make Daruma.
"Finally I started to feel like saying 'I'm not going to cry!'! (smiles) Because there are a lot of people in Tokyo who want a job in show business, I said I want to do this, more and more firmly." He won the audition for the role of Kikumaru Eiji in Tenimyu. Koji´s cat-like look and cheerful, agile nature made him the perfect match for this part, as fellow cast-mates and friends like Takiguchi Yukihiro, Sakurada Doori, Endou Yuuya commented, and later was praised by Konomi Takeshi, the author of the Original Manga The Prince of Tennis, for his portrayal of the role. But right at the start he faced obstacles: surrounded once more by new people and confronted for the first time with singing, dancing, acting and performing acrobatics - everything at once on a big stage. He worried endlessly about being up to the task, but with encouragement from his cast-mates Hiroki Aiba and Kouji Watanabe, a former gymnast, who practiced acrobatics with him, came the confidence. The teenager impulsively dyed his natural black hair in "Kikumaru-color" (a bright shade of red) and asked fans to call him Setomaru (a combination of his own surname and Kikumaru´s). On stage, he brought the hyperactive Eiji to life and earned full appreciation from fans and co-stars alike. In 4 musicals spanning a record total of 151 performances, Setomaru established his place in the hearts of fans in Japan and all over the world, until his Graduation alongside the other members of the 3rd Generation Seigaku on the September 9, 2007. It was a tearful parting, but Tenimyu helped him grow. "The experience taught me the excitement and terror of performing in front of an audience."
Koji´s D-BOYS logo is a chameleon in reference to his versatility - and he proved it. In late 2006 the young actor got chosen as Thursday's-face of Oha Suta, a children's TV show airing live every morning on TV Tokyo, as "Seto-kun". Since then, he has been a regular co-host once a week of the show next to Kōichi Yamadera and was even named as the most popular male person of the program by the TV viewer. But he is not just a boy in an old-fashioned green uniform - Seto-kun has the ability to change his persona with the use of his "Miracle Glasses". This way, Koji was able to act out his part in 8 different ways. After the winter 2006-2007 tour of Tenimyu - Absolute King Rikkai feat. Rokkaku ~ First Service, he was cast in spring 2007 as the bright greenhorn Kyoichi Segawa, a young gentleman host in Happy Boys, and his first leading part. The drama followed Makoto Tateno´s script, which was published alongside the TV Series in Manga-form. Mangaka Tateno created the character design originally based upon the actors, a very rare occurrence. Right after this series, the teenager was offered his first starring role in a movie, as the awkward salesman Chibe-kun, who tries to deal with the yaoi passion of his girlfriend in the crazy comedy Tonari no Yaoi-chan, a movie based on a real-life blog that centered on the 801-chan phenomenon.
In summer 2007 premiered D-BOYS STAGE Vol. 1 - the first stage play in which all D-BOYS participated. Most remarkable in the theater work was the final, a complex and well-choreographed battle scene depicting the last chapter of the legendary Shinsengumi, in which Koji portrayed Okita Sōji. Since July 2007 he made further appearances such as an ikemen on Fuji TV's Friday night show Thrill Night - United Ikemen, a celebrity entertainment show in which he was the youngest regular ikemen in November. The role of the hapless teenager Izamu Hayama in the tragic motion picture romance Tenshi ga Kureta Mono was Koji´s first cinema production in autumn 2007, and his first opportunity to speak Kansaiben. Afterwards he co-starred in winter 2007 as pawky kindergarten teacher with Aya Ueto and Yo Oizumi in the Golden Time TV Series Abarenbo Mama.
In January 2008 has he become one of Japan's tokusatsu heroes by starring as the shy and truthful but lonely Wataru Kurenai alias Kamen Rider Kiva in the newest Kamen Rider Series. Asked about the similarities between himself and his character, the teenager made comparisons to the time when he had once been a stranger too. "I have a lot in common with Wataru. I think I'm honest, like he is and when I came to Tokyo two years ago, trying to make it as an actor, I didn't knew anybody here and was a real loner."