Katsunari was born as heir to the Mizuno clan of Kariya. At 16, he killed 15 enemies in his first battle but was later disowned by his father after stealing money and killing the retainer who reported him. Placed on a 'hokoukamae' (employment blacklist), he wandered for 15 years, serving and abandoning famous lords including Hideyoshi, Kuroda Kanbei, and Kato Kiyomasa.