Kubo was born on June 26, 1977 in Hiroshima Prefecture, where his father worked as a town council member. In elementary school, he had already decided to become a manga artist, due to reading the manga "Saint Seiya". His first one-shot was "Ultra Unholy Hearted Machine", written for the "Weekly Shōnen Jump" in 1996. He wrote his first manga "Zombiepowder", which was also published in "Weekly Shōnen Jump" in 1999. It ran a short 27 chapters before being canceled in 2000. According to the author's commentary, Kubo was in a state of severe emotional trauma when he wrote it. Kubo later stated that he was not used to the magazine weekly serialization and used to pay more attention to his editor's comments rather than his own ideas.