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.

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 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.

Personality
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 to 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 tricked Naruto Uzumaki into stealing Konoha's Scroll of Seals, which contains a number of forbidden jutsu. He convinced Naruto to do it by appearing sympathetic towards him, and told him that it would help him graduate from the Academy, but, in actuality, he despised Naruto. While he intended to steal the scroll from Naruto and to use Naruto as a scapegoat in the meantime, Iruka eventually figured out what he was up to and quickly caught up with Naruto and Mizuki. Realizing that it would be harder to get the scroll from Naruto if he teamed up with Iruka, he decided that he had to get Naruto to not trust Iruka. Mizuki then revealed to Naruto the truth about his past and why the village despised him: that he is the container for the Nine-Tailed Demon Fox.

As Naruto was frozen in shock by this knowledge, Mizuki used this to his advantage by hurling a giant shuriken at the frightened Naruto. Iruka jumped in at the last second to block the shuriken with his own 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 had learned from the scroll to defeat Mizuki as punishment for harming Iruka, which inevitably exacerbated Mizuki's hatred for Naruto.

Mizuki Strikes Back
Although Mizuki initially seemed to have no specific use for the Scroll of Seals, the anime's filler arcs showed 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 was seen in a prison of the Leaf village. After leading a prison riot to escape from jail, Mizuki used the instructions depicted on a tattoo, given to him by Orochimaru, to create a potion. Before that, he visited his lover, Tsubaki, who claimed that Mizuki used to be a sweet man, but that, ever since he got involved with Orochimaru, her relationship with Mizuki had faltered. Mizuki knocked out Tsubaki, who he apparently did not love anymore, and travelled to a cave where he could find the ingredient to the potion.

After consuming the potion, Mizuki obtained enhanced strength and power, stripes along his body, and enlarged arms. Later, the potion gave him a tiger-like appearance, but he became much slower. Naruto managed to defeat him with the Rasengan, with Iruka noting that Naruto had surpassed him. Mizuki was yet another victim of Orochimaru's manipulation, and turned into an angry, shrivelled, and powerless old man when the potion dissipated, due to the fact that the ingredients were incomplete. Tsunade eventually returned him back to his normal state (although he wouldn't be able to be a ninja anymore) and interrogated him about the whereabouts of Orochimaru, but learnt nothing of any use.

Abilities
At the beginning of the series, Mizuki did not seem very skilled beyond proficiently wielding large shuriken. His abilities were expanded upon in the anime, where he was revealed to have always been considered a very clever and talented ninja with a knack for learning advanced jutsu. In truth, the reason he had 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 (although both cases were not too difficult, as Naruto was considered to be a little naive at the time, while the Legendary Stupid Brothers were simply unintelligent).

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 just barely survived.

He also has a special tattoo (a "gift" from Orochimaru) which was a prescription of 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 with 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, although he lost all his battle and physical prowess.

Trivia

 * Both Mizuki and Aoi Rokusho share similar traits:
 * Both appear friendly to better deceive others.
 * They both tricked their "respective" students (Naruto and Idate) into stealing an item in order to "promote" them. Naruto: The Scroll of Sealing to become a Genin and Idate: The Sword of the Lighting God to become a Chunin.