Zabuza Momochi

was a missing-nin from Kirigakure, and the first major villain of the series.

Background
In the past, Kirigakure had the practice of pitting Ninja Academy graduates against each other in a battle to the death for a final exam. When Zabuza, not yet a graduate himself, killed all one-hundred of that year's students, the practice was discontinued. Zabuza would thereafter be known as the "Demon of the Hidden Mist". His fame grew over the years, as he became both an ANBU and a member of the Seven Ninja Swordsmen of the Mist. At some point he discovered Haku, a child with a special kekkei genkai. He began training Haku to be the ultimate weapon. In the anime, Zabuza, is shown to not have helped in the village's defense from the Kaguya clan. Soon after this he attempted to kill the Mizukage in a failed coup d'état, and subsequently fled with Haku and some other followers.

Land of Waves arc
In order to gain funds for a second coup d'etat, Zabuza became a mercenary assassin for hire. As one of his jobs, he was hired by Gato to kill a bridge builder named Tazuna. When the Demon Brothers are sent to eliminate Tazuna and fail, Zabuza decides to take care of him personally. Upon tracking down his target, Zabuza finds that Tazuna is being protected by Kakashi Hatake and his students. Wanting to test his ability, Zabuza challenges Kakashi to a battle, and Kakashi complies by revealing his Sharingan.

Soon after the battle begins, Zabuza manages to catch Kakashi off guard and traps him with his Water Prison Technique. Needing to stay with Kakashi to keep him imprisoned, Zabuza sends a water clone to kill Tazuna and the rest of Kakashi's team, believing it to be a simple matter. Naruto Uzumaki, however, manages to sneak up on Zabuza, forcing Zabuza to release Kakashi. Freed, Kakashi and Zabuza resume their battle, though Kakashi's Sharingan quickly gives him the upperhand. Copying Zabuza's Water Release: Grand Waterfall Technique before he can use it, Kakashi manages to defeat Zabuza.

Before Zabuza can be killed by Kakashi, Haku appears disguised as a hunter-nin and throws two needles into Zabuza's neck, apparently killing him. Taking Zabuza's body away under the guise of needing to destroy it, Haku revives Zabuza, the needles having been used to paralyze him. Although Zabuza's life has been saved, he needs a week to recover, and he plans to kill Tazuna and Kakashi then.

A week later, Zabuza and Haku appear before Team 7 for a rematch. As Haku deals with Kakashi's students, Zabuza deals with Kakashi. Having been told how the Sharingan works by Haku, Zabuza blankets the area in a thick mist to avoid falling prey to the Sharingan's influence. In doing so, Zabuza is able to slash Kakashi with his sword, greatly damaging his opponent. Kakashi, however, allowed himself to be attacked, and summons his ninken to sniff out the blood now on Zabuza's sword to track and pin him down.

With Zabuza unable to fight back, Kakashi prepares to kill Zabuza with his Lightning Blade, and charges at Zabuza. Before the Lightning Blade can meet its mark, Haku appears as a human shield and takes the full force of the attack, fulfilling his promise of being a tool for his master. Freed as a result of Haku's intervention and not willing to allow the opening Haku has created go to waste, Zabuza tries to slash through Haku's body to kill Kakashi, though Kakashi is able to avoid it and disable Zabuza.

With Zabuza no longer able to battle, Gato arrives and revokes their agreement so that he can have his own henchmen kill Tazuna instead. No longer having a reason to kill Tazuna, Zabuza ends his battle with Kakashi, though Naruto is not satisfied by this. Angered that Haku died for Zabuza while Zabuza has shown no signs of remorse, Naruto lectures him on the feelings Haku had for him and the loyalty he felt towards Zabuza.

Brought to tears by Naruto's words, Zabuza reveals that he really did care about Haku and valued him higher than a simple tool. Wanting to repent for what he has done, Zabuza takes one of Naruto's kunai and charges through Gato's men to get to Gato, being stabbed multiple times along the way. Upon reaching Gato, Zabuza kills him, and collapses due to his injuries. As Gato's men flee, Zabuza asks Kakashi if he can be brought to Haku, to which Kakashi complies. Being by Haku's side in death as Haku was by his in life, Zabuza wishes that he could go to the same place as Haku, and Kakashi believes that they will be able to go to that place together. Team 7 buried them next to each other.

Abilities
Zabuza is a very vicious battler, preferring more direct attacks than deception with strong taijutsu skill. However, despite Zabuza's aggressive nature, he has shown to be a crafty ninja capable of misdirecting his opponent. His signature weapon is the Decapitating Carving Knife, an over-sized broad sword. The sheer size of this blade demonstrates great physical strength on Zabuza.

Likely in part due to him being from the Hidden Mist, Zabuza seems to prefer water manipulation techniques and has shown great skill in them. He is especially adept at the Hidden Mist Technique, mostly because it allows him to show his true forte: Silent Homicide Technique. His skill at it is considered unrivaled and has mastered it to such an extent, that Zabuza can strike his opponents without the need of sight simply by focusing on any slight sounds made by the opponent.

Trivia

 * Zabuza's surname "Momochi" may have been named by creator, Masashi Kishimoto, after the famous ninja, Momochi Sandayu.
 * Zabuza's sword is taken by Suigetsu Hozuki from his grave in Part II.
 * Despite his short appearance in the series, Zabuza is still a very popular character.
 * In some scenes of the anime, Zabuza does not have any eyebrows.

Quotes

 * "When I was your age, these hands were already soaked in blood."
 * "A shuriken? You'll never touch me with that!"
 * "You're nothing more than a monkey who copies my moves!!"
 * "You're right. Those little people and their little dreams, why should I care about them?! I have my own dream!"
 * "Have you forgotten... That I am a genius at silent killing, taking down the enemy by just their sound."
 * "When you've hovered between life and death so many times that it doesn't faze you, you may be called a ninja. When you've become so deadly, your profile has been entered into my bingo book then you may have earned the title ninja."