Haku is Zabuza's trainee and most trusted companion. He is an exile from the Hidden Mist Village and he has a very powerful advanced bloodline. Because of the many civil wars in the Hidden Mist Village, the advanced bloodlines are hated. When Haku's father found out that Haku and his mother possessed an advanced bloodline, he tried to kill them. He managed to kill Haku's mother, but Haku survived and killed him instead.
After killing his father, Haku realized that he was unwanted. He didn't have any reason to exist. Haku wandered the streets homeless and alone, but that changed when he met Zabuza. Zabuza was also an outcast of the Hidden Mist Village and he wanted revenge. In Haku he saw a tool with which he could carry out his vengeance.
Haku was extremely happy he had finally found a reason to live. He had finally found someone who loved him for his abilities, instead of hating him because of them. Haku was determined to give his life to Zabuza. He said it himself, "I am Zabuza-san's weapon... please keep me by your side as your tool."
Haku is an expert in acupuncture, and his weapons of choice are senbon. With his advanced bloodline, Haku is able to control water. His most powerful jutsu is the Demonic Ice Mirrors. This makes Haku a very talented and powerful ninja, who even surpasses Zabuza.