Rank: B-Missing ninja
Haku was a good-natured, compliant, and eager shinobi, believing that a person becomes truly strong when they have someone important to protect. Despite his talent for battle, and incredible abilities, Haku disliked needing to kill others in his role as a tool, and avoided doing so whenever possible. When forced to face an opponent, Haku did all that he could to avoid inflicting great harm, even putting them into a death-like state to prevent them from fighting further, as he did to Sasuke during their battle. Before he met Zabuza, he suffered from a belief that he was unnecessary, and served Zabuza because he wanted to feel useful.
Haku was born in a small, snowy village in the Land of Water, a land that had suffered from war not long before. In this war, different sides battled each other using ninja, some of whom possessed kekkei genkai. After the war ended, memories of the horrific battles still lingered in the minds of the people. Those with kekkei genkai quickly became feared and hated for their abilities, out of concern that their existence would only bring about more war.
Haku's father and mother were simple farmers, and they lived a peaceful life. They loved each other, and were kind to their child. Unfortunately, this all changed one day. Haku's mother was a carrier of a kekkei genkai: Ice Release. She hid this fact from her husband, hoping that the love and peace that was shared in their small family would last forever. One day, Haku discovered the ability to manipulate water. Amazed by this, Haku proudly showed this to his mother, who was horrified by what she saw, and harshly scolded Haku for displaying his ability. Unbeknownst to his mother, Haku's father had seen everything. When Haku's father discovered that his wife and child possessed a kekkei genkai, he assembled a small mob of villagers, and killed his wife. He would have done the same to Haku, but before he could, Haku killed them.
Orphaned, Haku became a child who was wanted by no-one and was forced to take to the cold streets and rummage through trash bins for scraps of food, even sometimes having to fight off the wild dogs that roamed the streets. In time, he was found by Zabuza Momochi, who asked Haku to become his "weapon," which meant to become a dedicated kekkei genkai shinobi for Zabuza. Haku readily accepted this role, due to the purpose it gave him, devoting his life to becoming the ultimate weapon for Zabuza. Zabuza then trained Haku in the ways of the shinobi, as well as every fighting technique that he knew.
In the anime, it was revealed that Haku and Zabuza met Kimimaro some time after Zabuza cast aside the Land of Water and set out with Haku. Haku appeared to sympathize with Kimimaro for his loneliness and lack of purpose by looking into his eyes. However, he was forced to keep moving by Zabuza, and leave Kimimaro be.
When Zabuza was about to be killed by Kakashi Hatake, Haku appeared, disguised as a hunter-nin, playing the part by appearing to kill Zabuza. He took the body, and, elsewhere, revived him, leaving Zabuza to rest a week before he could have a rematch with Kakashi. During this time, he met Naruto, and had a conversation with him about their purposes in life and those they care about.