Zabuza Momochi

was a missing-nin from Kirigakure, and the first major antagonist 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 student 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. He also killed the Konohagakure ninja Kumade Toriichi. At some point he discovered Haku, a child with a special kekkei genkai and began training him to be the ultimate weapon. In the anime, Zabuza was shown to not have helped in the village's defense against 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, such as the Demon Brothers.

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 Gatō to kill a bridge builder named Tazuna. When the Demon Brothers were sent to eliminate Tazuna and failed, Zabuza decided to take care of him personally. Upon tracking down his target, Zabuza found that Tazuna was being protected by Kakashi Hatake and his students. Wanting to test his ability, Zabuza challenged Kakashi to a battle, and Kakashi complied by revealing his Sharingan.

Soon after the battle began, Zabuza managed to catch Kakashi off guard, and trapped him with his Water Prison Technique. Needing to stay with Kakashi to keep him imprisoned, Zabuza sent a water clone to kill Tazuna and the rest of Kakashi's team, believing it to be a simple matter. Naruto Uzumaki, however, managed to sneak up on Zabuza, forcing Zabuza to release Kakashi. Freed, Kakashi and Zabuza resumed their battle, though Kakashi's Sharingan quickly gave him the upper hand. Copying Zabuza's Water Release: Grand Waterfall Technique before he could use it, Kakashi managed to defeat Zabuza.

Before Zabuza could be killed by Kakashi, however, Haku appeared, disguised as a hunter-nin, and threw two needles into Zabuza's neck, apparently killing him. Taking Zabuza's body away, under the guise of needing to destroy it, Haku revived Zabuza, the needles having been used to paralyze him. Although Zabuza's life had been saved, he needed a week to recover, and he planned to kill Tazuna and Kakashi then.

A week later, Zabuza and Haku appeared before Team 7 for a rematch. As Haku dealt with Kakashi's students, Zabuza dealt with Kakashi. Having been told how the Sharingan worked by Haku, Zabuza blanketed the area in a thick mist, to avoid falling prey to the Sharingan's influence. In doing so, Zabuza was able to slash Kakashi with his sword, greatly damaging his opponent. Kakashi, however, allowed himself to be attacked, and summoned 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 prepared to kill Zabuza with his Lightning Blade, and charged at Zabuza. Before the Lightning Blade could meet its mark, however, Haku appeared as a human shield, and took 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 had created to go to waste, Zabuza tried to slash through Haku's body to kill Kakashi, though Kakashi was able to avoid it and disable Zabuza.

With Zabuza no longer able to battle, Gatō arrived, and revoked their agreement so that he could have his own henchmen kill Tazuna instead. No longer having a reason to kill Tazuna, Zabuza ended his battle with Kakashi, although Naruto was not satisfied by this. Angered that Haku died for Zabuza, while Zabuza had shown no signs of remorse, Naruto lectured him on the feelings Haku had for him, and the loyalty he felt towards Zabuza.

Brought to tears by Naruto's words, Zabuza revealed that he really did care about Haku, and valued him higher than a simple tool. Wanting to repent for what he had done, Zabuza took one of Naruto's kunai, and charged through Gatō's men to get to Gatō, being stabbed multiple times along the way. Upon reaching Gatō, Zabuza killed him, and collapsed due to his injuries. As Gatō's men fled, Zabuza asked Kakashi if he could be brought to Haku, to which Kakashi complied. Being by Haku's side in death, as Haku was by his in life, Zabuza wished that he could go to the same place as Haku(Presumably heaven as he believes he will go to hell). Kakashi believed that they would be able to go to that place together. Team 7 buried them next to each other, with the Decapitating Carving Knife stabbed into the ground.

Abilities
A Water-user and a sword-user, Zabuza was a very vicious battler, preferring more direct attacks than deception, along with strong taijutsu skill. However, despite Zabuza's aggressive nature, he had been shown to be a crafty ninja with enough skill to kill opponents through zero visibility, capable of misdirecting his opponent.

Weapon Skills
His signature weapon was the Decapitating Carving Knife, an oversized broad sword. The sheer size of this blade demonstrated great physical strength on Zabuza's part. Apart from handling it very skillfully, Zabuza could also throw it spinning to several enemies. He threw it with power enough to stick it into a hard tree trunk.

Water Release
Likely, in part, due to him being from the Hidden Mist, Zabuza seemed to prefer water manipulation techniques, and had been shown to have great skill in them. He was especially adept at the Hidden Mist Technique, mostly because it allowed him to show his true forte: Silent Homicide Technique. His skill at it was considered unrivalled, and he had mastered it to such an extent that Zabuza could strike his opponents without the need of sight, simply by focusing on any slight sounds made by the opponent.

Trivia

 * Zabuza's surname, "Momochi," might have been chosen by creator, Masashi Kishimoto, after the famous ninja, Momochi Sandayu.
 * Zabuza's sword was taken by Suigetsu Hozuki in Part II.
 * Despite his short appearance in the series, Zabuza was still a very popular character.
 * In some scenes of the anime, Zabuza did 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!"
 * (To Kakashi) "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 that your profile has been entered into my bingo book, then you may have earned the title ninja."
 * (To Naruto) "Your words cut deep, deeper than any blade"
 * (Last words to Haku) "You were always at my side, the least I can do is to be beside you at the end. I know I can not be, but I wish I could go to where you have gone. How I wish I could join you there, Haku."