Mizuki is a childhood friend and was a former fellow Ninja Academy instructor of Iruka Umino. He later becomes the first antagonist of the series. Mizuki's childhood friend was Iruka Umino. Mizuki often stood up for Iruka when Iruka felt bad about losing his parents. However, he began to hate Iruka when other people began treating Iruka better than him, including their friends, the Third Hokage, and even people they didn't know. Eventually, his kindness towards Iruka became only a front to make other people notice him and find ways to discreetly keep Iruka miserable. Despite obtaining the rank of Chūnin and status as an Academy Instructor, such status was not enough to satisfy his ego. Eventually, he decided he would do whatever it took to rise to glory, including sacrificing allies on missions.