Kojima Yatarō served as a bushō during the Sengoku period. He was a retainer of the Uesugi clan of Echigo Province.
Yatarō is referred to in a military chronicle as “Kojima Yatarō the Demon.”
Yatarō is deemed to have originated from the area of Myōkō-Takahara in Echigo and, from an early age, served as a close associate of Uesugi Kenshin, the sengoku daimyō of Echigo. Known as a hero of unparalleled strength, he was feared as Kojima the Demon. Yatarō, however, is not listed in the military records of the Uesugi so there are questions as to whether he in fact existed. There were many individuals with the Kojima surname in the Uesugi family so there is a theory that he was a fictitious creation from this group.
There are many colorful legends regarding Yatarō including that he chastised a large monkey kept by Ashikaga Yoshiteru, the thirteenth shōgun of the Muromachi bakufu (similar to an anecdote regarding Ōta Dōkan). In another account, when serving as a messenger to the Kai-Takeda clan, he struck down a fierce dog that attacked him. Next, when Yamagata Masakage engaged Yatarō in a one-on-one duel, Masakage noticed that Takeda Shingen’s eldest son, Takeda Yoshinobu, was in a precarious situation. When Masakage then made a request to Yatarō to pause their bout so he could rescue the son of his lord, Yatarō obliged and earned a debt of gratitude. Finally, for his contributions at the Battle of Kawanakajima, he was extolled as being, in substance and in form, a brave warrior.
According to legend, Yatarō served as the lord of Otoyoshi Castle in Echigo. He was killed in action on Mount Tenjin located in the town of Tochio-Ōno in the city of Nagaoka in Niigata Prefecture.
His grave is at the Eigan Temple in the city of Iiyama in Nagano Prefecture.