Some TMNT stuff really isn't for little kids.
Gosei Hattori is the last heir of the Gosei clan, a samurai family with a long tradition in the history of Japan. But his father, Gosei Kenji, grew up more according to the values of the western world and wanted nothing more to do with the "outdated" heritage of his family out of interest of money; therefore, Kenji moved to New York and built himself a crime syndicate which brought him a life of prosperity, and tried to convince his son again and again that it would be his duty, to be his successor as managing director of the syndicate, even though Hattori had no interest in it.
Gosei Hatsumi, Hattori’s grandfather, meanwhile was dying in Japan. Because he feared that Hattori would deviate from his path as the heir of the clan, he sent his spirit in the search for an assistant and found him in Splinter. After Splinter had learned of the circumstances he and Hatsumi exchanged their bodies and Hatsumi led (in Splinters body) the Turtles to Hattori, to give him his heritage. However, Donatello and Raphael stumbled unexpectedly onto an assassination plot against the Gosei, which had been instigated by two greedy subordinates of Kenji, After they return to Hatsumi with this news, he orders them to kill the assassins. Unfortunately, the Turtles miss one of the killers, and he guns for Hatsumi. Kenji steps in the way and takes the bullets meant for his son as Leonardo stops the killer before he could shoot Hattoir at the last moment thus saving Hattori and allowing Hatsumi to pass on access to the mystical ways of the Gosei heritage to his grandson.
Since his heritage had become accessible, Hattori gave up his father's syndicate and lived for some years by the way of the warrior. Later, however, he was in danger, as the inter-dimensional magician Savanti Romero traveled to the Japan of the 14th century and there attacked the mind of Hattori's ancestor Gosei Oraga with his black magic, to usurp the mystical knowledge of the Gosei Clan. This process also began to rob the mind of Hattori over the abyss of time, and therefore he sought out the Turtles, to ask them for help. When the time guardian Renet appeared shortly after his arrival at the Jones Farm, the opportunity arose to thwart Savanti's plan and to restore Hattori's mental state. Later the Turtles helped Hattori to wipe out the evil legacies of his father's from his company.
- Hattori's first name is shared with Hattori Hanzō, a historically proven ninja who lived in the 16th century.