Koji Seto was inspired to become an actor when he saw Tsumabuki Satoshi, a fellow Fukuoka-born actor, starring in the TV drama series "Orange Days". He was a student at the Fukuoka Actors School, which is part of the renowned Himawari Acting School. He took on his first role 2005 in the TV Drama "Rocket Boys" and won in the same year a audition for Watanabe Entertainments Acting Group D-BOYS. It was also the year when he moved on his own to Tokyo, with just 17 years. He successfully took on several roles in TV, movie and stage productions. In 2008 he became the first actor who appeared in two leading parts, aired on TV at the same time, starring as Wataru Kurenai in "Kamen Rider Kiva" and Hiro in the TV-drama adaption of "Koizora". A wide range of different leading parts followed, like 2010 as mentally disabled teenager Shuhei in "Rinne no Ame", a drama that won Fuji TV's 21th Best Young Screenwriter Award, and as Yuta, the captain of a male-tumbling team in the sport drama "Tumbling". 2011 he starred as the brave young Samurai Mori Ranmaru, alongside Etsushi Toyokawa as Nobunaga Oda, in NHK's Taiga Drama "Princess Go". 2012 appeared he in the sequel of the "Ring" movie series "Sadako 3D" as Takanori Ando. Also 2012, Director Shiota Akihiko choose him to star in the tv drama "Spy Tokku", a story about a spy that underwent a face transplantation when he started his job with only 19 years.