Like most Ninningers except Takaharu, he initially refuse to become a Ninninger, having no courage until Takaharu encouraged them. At first he tried to use textbooks as guidances but having none them helped him in against Youkai Kasha, he decided to use his experience instead, taking Takaharu as an example for not thinking over his actions first. One of Yoshitaka Igasaki's successors, Nagi was one of his grandchildren chosen to become the Ninningers as KiNinger, the Agile Ninja. He was called by Tsumuji Igasaki, his uncle and Takaharu and Fuuka's father to their dojo, which much later turned into a pile of ruins by Hitokarages.
After the final battle against Gengetsu Kibaoni, Nagi studies to get into university.