Zabuza is a missing-ninja from the Village Hidden in the Mist. He is also one of the seven swordsmen of the mist earning him the nickname "Demon of the Hidden Mist" and the first major antagonist of the series.
The Village Hidden in the Mist use to have a deadly ninja academy. The only graduates who became ninjas were those who killed all of their peers and survived the training. Zabuza (who wasn't even an academy student yet) slayed one-hundred academy students and became a ninja. His massacre was so terrible the practice was discontinued, Zabuza was the last graduate of that method.
Later in his ninja career he raised himself up to the ranking ANBU and became a member of the Seven Swordsmen of the Mist (it is unknown which he achieved first). By now he was a well-known ninja whose name was in the bingo books.
Eventually he found an orphan, Haku. Impressed by the powerful kekkei genkai Haku wielded Zabuza decided to train him to become his ultimate weapon. After training Haku and gathering a small fleet of ninjas Zabuza attacked his country's kage, the Mizukage. The assassination failed and Zubuza was forced to retreat with Haku, the Demon Brothers, and other ninjas who tried to help him. After this Zabuzu and his gang became missing-nins-for hire. One of their employers was Gato.
Zabuza, "The Demon of the Mist" was an assassin hired by the multi-millionaire/crime boss Gato. His job was to kill the bridge maker Tazuna, whom happened to be guarded by Kakashi and team Seven. Zabuza first appeared to attempt his assassination right after Team 7 defeated the Brothers of the Mist. The ensuing fight between Zabuza and Kakashi was rather long and involved everyone doing something to help fight off the demon. Sakura guarded Tazuna as Kakashi fought him out on a lake. Naruto and Sasuke were busy dealing with one of Zabuza's Water Clones, which proved to be too hard for either of them to defeat.
Team 7 didn't realize how much trouble they were in until Kakashi was trapped in Zabuza's water prison technique. Naruto and Sasuke had to develope an elaborate plan involving transformation techniques and diversions, ultimately forcing Zabuza to release his water prison technique on Kakashi. Zabuza and Kakashi battle continued until Kakashi pinned him to a tree. When Kakashi was finally ready to kill Zabuza, a mysterious masked figure finished the job. Kakashi checked to see if Zabuza had a pulse, and failed to find one. The figure told them that many secrets lie within this ninja, and could be very dangerous if fallen into the wrong hands.
The figure took Zabuza with him and Kakashi noted that he must have been a tracker ninja for the Mist village. After all, Zabuza was wanted in his home city. It was later discovered that the mysterious masked ninja worked under Zabuza and was actually Haku. He tricked Team 7 so they would not kill his master.
After having everyone train, Kakashi, Sakura, and Sasuke headed off to the bridge that Tazuna was close to finishing. They soon learned that Zabuza and Haku were waiting for them. Sasuke took on Haku while Kakashi took on Zabuza. The fight raged on until Naruto interrupted with bold introduction. Soon both Naruto and Sasuke were trapped inside one of Haku's many deadly Jutsu, the "Demonic Ice Mirrors," a huge room of ice mirrors that Haku was able to make due to his kekkai genkai. Meanwhile, Zabuza was hiding in his mist and fighting Kakashi silently and swiftly.
As the battle raged on Kakashi soon caught Zabuza and ensnared him with his tracking dogs. He was about to deliver a fatal chidori to the Demon of the Hidden Mist when Haku jumped in front of the attack, dying to save his master.
"You were always at my side, the least I can do is to be beside you at the end. I know it cannot be, but I wish I could go to where you have gone. How I wish I could join you there, Haku."
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