Mizuki (ミズキ) is a childhood friend and 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 Chunin 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.
Originally, Mizuki appeared to be very polite and kindhearted, and was described to be a sweet man by his former lover, Tsubaki. In reality, he does not care for anything or anyone beyond his own rise in power. He takes pride in the fact that he is not bound by things like morals or teamwork and is willing to sacrifice whatever he has to in order complete his goals. His desire for power seems to come from what appears to be an inferiority-complex as he cannot stand having anyone look down on him. In what is perhaps an attempt to make him even more of an antithesis to Naruto, Mizuki's least favorite food is ramen, which happens to be Naruto's favorite. Mizuki also has a particular hatred for Naruto.