Mizuki

is a childhood friend and fellow Ninja Academy instructor of Iruka Umino. He later becomes the first antagonist of the series.

Background
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 a 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.

Personality
Originally, Mizuki appeared to be very polite and kindhearted. 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.

Part I
At the series' start he tricks Naruto Uzumaki into stealing Konoha's forbidden scroll that contains a number of forbidden jutsu. He convinces Naruto to do it by appearing sympathetic to him, but actually despises Naruto. While he intends to steal the scroll from Naruto and use Naruto as a scapegoat in the meantime, Iruka eventually figures out what he is up to catches up to Naruto and Mizuki. Realizing that it would be harder to get the scroll from Naruto if he teamed up with Iruka, he decides he has to get Naruto to not trust Iruka. Mizuki then reveals to Naruto the truth about his past and why the village despises him: that he is the container for the Nine-Tailed Demon Fox.

As Naruto is shocked by this knowledge, Mizuki uses this to his advantage by hurling a giant shuriken at a scared Naruto. Iruka jumps in at the last second to block the shuriken with his back, wounding him. Later, because of Iruka's kind, caring words and selfless action, Naruto had the courage to use the Multiple Shadow Clone Technique he learned from the scroll to defeat Mizuki as punishment for harming Iruka, which inevitably exacerbates Mizuki's hatred for Naruto.

Mizuki Strikes Back
Although Mizuki initially seems to have no specific use for the scroll of sealed jutsu, the anime's filler arcs show that, as another supporter of Orochimaru, he intended to give the scroll to his master. During the filler arcs, a much bigger and more muscular Mizuki is seen in a prison of the Leaf village. After leading a prison riot to escape from jail, Mizuki uses the instructions depicted on a tattoo given to him by Orochimaru to create a potion. Before that, he visits his lover, Tsubaki, who claims that Mizuki used to be a sweet man, but ever since he got involved with Orochimaru her relationship with Mizuki has faltered. Mizuki knocks out Tsubaki, who he apparently does not love anymore, and travels to a cave where he can find the ingredient to the potion.

After consuming the potion, Mizuki obtains enhanced strength and power, stripes along his body, and his arms are enlarged. Later, the potion gives him a tiger-like appearance, but he gets much slower. Naruto manages to defeat him with the Rasengan, with Iruka noting that Naruto has surpassed him. Mizuki is yet another victim of Orochimaru's manipulation, and turns into an angry, shriveled, and powerless old man when the potion dissipates due to the fact that the ingredients were incomplete. Tsunade eventually heals him back to his normal state (although he won't be able to be a ninja anymore) and interrogates him about the whereabouts of Orochimaru, but learns nothing of any use.

Abilities
At the beginning of the series, Mizuki did not seem very skilled beyond proficiently wielding large shuriken. His abilities are expanded upon in the anime, where he is revealed to have always been considered a very clever and talented ninja with a knack for learning advanced jutsu. In truth, the reason he never advanced beyond Chunin was because there were suspicions on his conduct while in the field. He was also shown to be a very cunning manipulator from how he so easily deceived Naruto and then later the Legendary Stupid Brothers.

After Mizuki's life in prison, he seemed to be much more muscular and had an extreme amount of strength. During his battle with Iruka, he demonstrated a new mastery of a random assortment of jutsu. While mostly improvised, he proved very capable of using them in clever combinations that Iruka barely survived.

He also has a special tattoo (a "gift" from Orochimaru) which was a prescription on a potion that initially transformed him into a muscle-bound behemoth of himself with tremendous raw speed and strength. It also allowed him to transform further into a tiger-like being with even greater strength but much weaker speed. But since the potion was incomplete, his body was eventually destroyed, although Tsunade managed to save his life and restore his body, though he lost all his battle and physical prowess.