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.
Zabuza Momochi means "he who never strikes twice".