Zabuza Momochi

Zabuza Momochi is a Nukenin (missing-nin) from Kirigakure (Village Hidden in the Mist). He attempted to kill the Mizukage in a failed coup de'tate against the Land of Water. Zabuza has been called the Demon of the Hidden Mist because of his actions as a child. While still living in Kirigakure he killed all the ninja academy graduates and the masacre ended Kirigakure's tradition of setting their student against each other in death matches. After leaving he discovered Haku, a child with a special kekkei genkai that allowed him (Haku) to manipulate water, ice, etcetera by using chakra. He began training Haku to be the ultimate weapon.

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.

Wave 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. 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 occassion, 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.