Zabuza Momochi

is a missing-nin from Kirigakure, and the first notable villain of the series. Before leaving Kirigakure, however, Zabuza was a member of Kirigakure's ANBU.

Background
Zabuza has been called the Demon of the Hidden Mist; this is because during his childhood, he had killed over 100 of the ninja academy graduates attending their final exam. In the exam each student would be pitted against the other in a battle to the death. The massacre ended this tradition immediately. He attempted to kill the Mizukage in a failed coup d'état, and subsequently fled. Just before leaving the Village he discovered Haku, a child with a special kekkei genkai that allowed him to combine the wind and water elements to bring forth ice. He began training Haku to be the ultimate weapon.

Weapons & Abilities
His weapon of choice is the giant, an oversized, broad sword that is later wielded by Suigetsu Hozuki The circle cut out of the top and the semicircle near the handle, as demonstrated by Suigetsu later in the manga, seem to fit it's purpose of decapitation aptly.

Likely in part due to him being from the Hidden Mist, Zabuza seems to prefer water manipulation techniques. He is especially adept at the Hidden Mist Technique, mostly because it allows him to show his true forte: Silent Homicide Technique.

Land of Waves Arc
In order to gain funds for a second attempt on the life of the Mizukage, Zabuza took a job where he was supposed to kill a bridgebuilder named Tazuna. Initially sending mere chûnin after him, only to have them thwarted by Kakashi and team, he told Gato that he would deal with them personally. He made a grand appearance, throwing his Zanbatō so that it stuck into a tree, and alighting on top of it.

In the first part of this battle, he built up a large amount of chakra, and unleashed his signature "Kirigakure no jutsu", or the Hidden Mist Jutsu. Effectively freezing Team 7, he attempted to kill Tazuna right there. However, Kakashi put a stop to this, and having copied the mizubunshin Zabuza had made, proceeded to stab it, dispelling the jutsu. Another mizubunshin came up behind Kakashi, and sliced him in half, but revealing the fact that his Sharingan had already copied the water clone technique.

Zabuza unravelled the final mizubunshin jutsu, and from behind Kakashi, swung a powerful horizontal strike, which Kakashi ducked. However, Zabuza followed this up with a kick that sent Kakashi sprawling into a nearby lake. Realizing his peril too late, Kakashi was trapped in Zabuza's Water Prison jutsu. However, with a good plan on behalf of Naruto Uzumaki, the genin from Team 7 were able to break the jutsu. With Kakashi freed, it was short work for his Sharingan to copy all of Zabuza's techniques, causing him to become increasingly desperate. Just as Zabuza appeared to be at Kakashi's mercy, Haku arrived, throwing two senbon into Zabuza's neck and sending him into a deathlike trance, in order to rescue him by tricking Team 7.

From the trance that he is put in, Zabuza's strength is sapped, forcing him to wait for a week to recover. Meanwhile, Kakashi also felt the effects of the extended use of his Sharingan, also needing some time to recover. Within this time frame, the bridge was nearly completed, and the genin of Team 7 took some time to train their chakra manipulation techniques. However, Zabuza was not out of the picture yet. He showed up at the bridge, slaughtering the workers before anyone else arrived. Kakashi had to once more face the Demon of Kirigakure.

Sending Haku after Sasuke and Naruto, Zabuza was able to focus exclusively on Kakashi. Using his Hidden Mist technique again, he succeeded in striking Kakashi several times, drawing a decent amount of blood. But, upon feeling the presence of the Demon Fox's chakra, Kakashi realized he was out of time. He performed the first Summoning technique seen in the manga, which was called at the time "Earth style: Fanged vengeance technique". His ninja dogs, attracted to his blood on Zabuza and his weapons, were able to effectively get hold of Zabuza, based on scent rather than sight in the thick mist, which unravels once Zabuza is caught.

Kakashi then began charging a Lightning Blade. Attempting to kill Zabuza with it, Haku intervenes, creating a crystal ice mirror, which he almost arrives at. However, he succeeds in stopping Kakashi's blow, costing him his life; when the raikiri hits Haku, the summoned ninja dogs disappear. Zabuza, now free to move, attempts to cut through Haku to hit Kakashi, not realizing that Haku was already dead. Now able to see once more, Kakashi is able to use his greater speed to counter Zabuza's attacks. He ends it by stabbing a kunai through the triceps of each of Zabuza's arms. At this point, the fight is interrupted by Zabuza's employer, Gato who arrives with his thugs.

Gato reveals at last his true plans. He never intended to pay Zabuza for the assassination. Instead, he took advantage of his Missing-nin status, because, as he put it, no one complains when he betrays and murders them. Noticing the dead Haku, he went over and kicked at his corpse, exclaiming that it was revenge for Haku crushing his arm when he visited the two of them while Zabuza was recovering. Naruto, astounded by Zabuza's apparent indifference, berated Zabuza about the devotion Haku had for him, saying that it's too cruel for him to have merely viewed Haku as a tool. Zabuza stops Naruto, and reveals one of the main themes Kishimoto presents in this sequence: no matter what they say, shinobi are people after all, with human emotions. With that, he asks for Naruto's kunai. Holding it in his mouth, he tears through Gato's mob, stabbing and then beheading the crime boss. With that, he falls to the ground. In the anime, Zabuza repeatedly slashes him up to the point where Gato falls into the water.

After Naruto and Kakashi cause the mob to disperse with a shadow doppelganger from Naruto and a normal doppelganger from Kakashi (although it is titled as "shadow doppelganger" in the manga, Kakashi himself stated that he could not manage anything solid in his weak state after the fight), Zabuza asks to be laid by Haku. It begins to snow over them both, prompting Zabuza to ask if Haku is crying. He apologizes to Haku, and thanks him for his faithful service, and then he dies. Team 7 buried them next to each other.

Years later, Sasuke would return with Suigetsu, who would take Zabuza's sword from the site of his grave.

Flashbacks
Zabuza never reappears in flashbacks save for one occasion, where Kimimaro Kaguya recalls an attack by his clan that resulted in their almost complete destruction. He probably was still a shinobi at Kirigakure, since he is seen dressed in the Mist's flak jacket. However, Zabuza did not return to the Mist village to assist in its defense, hinting that Zabuza probably was in the process of becoming a missing-nin at the time. It is also possible that Zabuza's inaction to help his village is the catalyst of his missing-nin status. Haku was also present in the flashback.

Quotes

 * "Ninja art: Hidden Mist Jutsu!"
 * "Water Clone Jutsu!"
 * "A shuriken? You'll never touch me with that!"
 * "Water Style: Water Dragon Jutsu!"
 * You're nothing more than a Monkey who copies my moves!!
 * "Can. . . Can you see into the future?"
 * You're right those little people and their little dreams, Why should I Care About them?! I have my own dream!!

Trivia

 * Zabuza's sword's handle can come off, making it easier to carry around.
 * The fights with Zabuza happened very early in the manga. Masashi Kishimoto would alter his ideas somewhat after that, causing there to be slight discrepancies between this and later explanations/appearances of certain moves.
 * The most notable differences are in the Raikiri, and the "Art of the Shadow Doppelganger Kakashi Style".