Zabuza Momochi

Zabuza Momochi is a missing-nin from Kirigakure, and the first notable villain of the series.

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 there final exam. In the exam each student would be pitted against the other in a battle to the death. The massacre had ended the village's tradition of pitting their student against each other in battles to the death. He attempted to kill the Mizukage in a failed coup de'tate against. After leaving the Village he discovered Haku, a child with a special kekkei genkai that allowed him to manipulate the ice element. He began training Haku to be the ultimate weapon.

Weapons & Abilities
His weapon of choice is a Zanbato, which translates into "horse slayer" as it was intended to cleave the head of a horse off with it's massive weight and size. He seems to prefer water jutsu, this is probably due to him being from the Hidden Mist. Although the zanbato's name was never mentioned at all, Suigetsu mentioned that its name is Executioner.

Mission to the Land of Waves Arc
He took a job where he was supposed to kill a bridgebuilder named Tazuna, after finding Tazuna he failed his first attack because of Team 7 who were hired to protect the bridge builder. Kakashi held him off, eventually having to depend on his Sharingan but soon is captured and sequentially rescued by Naruto and Sasuke. Before Kakashi could deal the death blow to Zabuza, Haku came to the rescue, faking Zabuza's death. The fake death prevents Zabuza from reentering combat for a week.

After a gruesome battle on the incomplete bridge, Haku sacrificed himself to save Zabuza who at first felt no remorse for his death stating that he was only a tool. Zabuza killed Gato afterwards, who had by then admitted that he hadn't been planning on following through with his end of the deal. While he laid dying Zabuza asked Kakashi to bury him next to Haku.

Flashbacks
Zabuza never reappears in flashbacks save for one occasion, where Kimimaro recalls an attack by his clan that resulted in their almost complete destruction. He probably was still a jonin 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.