Mizuki

is a childhood friend and fellow Ninja Academy instructor of Iruka Umino.

Background
Mizuki's childhood friend was Iruka Umino despite what he thinks of him now. Often Mizuki stood up for Iruka when Iruka felt bad for loosing his parents.

It is also revealed that Mizuki once killed a Konoha Jonin by sufficating him.

Personality
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 hate of Naruto Uzumaki.

Introduction of Naruto
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, considering him no different than the demon fox. While he intends to steal the scroll from Naruto and use Naruto as a scapegoat in the meantime, Iruka eventually figures out what he's up to and the two are drawn into battle, during which he, in a crazed rage, throws a giant shuriken at a scared Naruto. Iruka jumps 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. 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 Strikes Back
Mizuki's lover, Tsubaki, claims that Mizuki used to be a sweet man, but ever since he got involved with Orochimaru her relationship with Mizuki has faltered. After she interrogates Mizuki about having a grudge against one who put his life on the line during Orochimaru's attack, Mizuki knocks out Tsubaki, who he apparently does not love anymore.

Although Mizuki initially seems to have no specific use for the scroll of sealed jutsu, the anime's filler arcs show that, as a former student 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 revolt to escape from jail, Mizuki uses the instructions depicted on a tattoo given to him by Orochimaru to create a potion.

After consuming the potion, Mizuki uses it enhance his Curse Mark where he has enhanced speed and power, stripes and his arms are enlarged. Then, the potion gives Mizuki a tiger-like appearance. 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 and interrogates him about the whereabouts of Orochimaru, but learns nothing of any use.

Abilities
At the begining of the series, Mizuki seemed to be not very skilled due to being easily defeated by Naruo's Multiple Shadow Clone Technique. After Mizuki's life in prison, he seemed to be much more muscular and had an extreme amount of strength. He also did not fall for Naruto's trick earlier in the series now. He also seemed to have a special cursed seal now which would transform him into a tiger-like mode. Because of his overdoing of this though, he became incarcerated.