Before Haku was born, civil war raged throughout the Land of Water, and indeed, most of the surrounding countries as well. The greatest weapon one could have during this time of war were people with Kekkei Genkai.
Kekkei Genkai (“Bloodline Limits”) are abilities passed down genetically in specific clans. Most clans have developed special jutsu related to their kekkei genkai. Kekkei genkai and their related jutsu can't be taught or copied by others, as they require the specific trait to work.
Because they were used as a deadly, sometimes horrifying weapon, those with Keekei Genkai came to be associated with pain, death, fear, and war. After the wars were over, many of those with any bloodline ability were hunted down and systematically slaughtered. It's after the wars are over that Haku's brief story started.
Haku was born in a village in the Land of Water where it was always snowing. His father, mother and he lived on a farm (what did they farm? I don't know. Maybe snow? It's never explained what a farm is doing in a place that is always cold), and for a time they were happy. One day, a very young Haku discovered that he could do amazing things with water and wind. So excited and pleased, he quickly rushed off to show his mother. She was not nearly as pleased as Haku was and slapped him, telling him to never, ever do anything like that again. She then hugged him and started to sob.
Haku's mother had a kekkei genkai, one that involved water, wind and ice, but she had kept it a secret for many years, from her husband, son and friends. And now, with her son showing the same abilities, she was frightened for their lives. She had good reason to be.
Unknown to them both, Haku's father had seen the entire exchange and ran back to town to get together a band of men to return to his own and kill his wife and child. Armed with farming tools and torches, his father and a large group of men from the village murdered Haku's mother before his eyes and advanced on him. In desperation, Haku unleashed his gift. A short while later, he staggered out of the ruins of his house, having killed all who were there with his budding Kekkei Genkai. He doesn't remember how long he wandered in the snow, but he eventually came to a town where, emotionally shattered from realizing that nobody wanted him, or would ever want him, he became a beggar. Living off the streets, doing whatever he could to survive, Haku spent many of his younger years simply wandering from place to place within the Land of Water until the day he met Zabuza Momochi.
Sitting on a bridge in the snow while dressed in thin rags was how Haku met Zabuza-san. The tall, intimidating-looking ninja looked down at him and scoffed. "You'll never be wanted. You'll die there, alone and unloved." Haku heard none of this, but rather, smiled warmly up at him and told him "Your eyes are just like mine." For whatever reason, this hit a mark in Zabuza and he decided to take Haku along with him. Told to not expect anything like companionship or affection, Haku was told that Zabuza only needed a tool, one that would be strong and only his. Haku agreed readily, giving his unfailing loyalty to Zabuza-san. He was just so happy to be wanted by someone that he had no problem becoming someone's tool, an object.
After Zabuza’s failed coup d'état, the two fled the Land of Water. While traveling, Zabuza began to train Haku in the ways of a ninja. Haku was a quick study; in little time at all, he had surpassed Zabuza in many areas, even going so far as to perform one-handed seals, a feat no other ninja had ever accomplished. Haku became an expert in hand-to-hand combat, acupuncture, medical skills, all while carefully refining his Kekkei Genkai. It was during their traveling that the pair ran into a man named Gado.
Gado offered them a large sum of money to remove a certain person that had been bothering his business and Zabuza agreed to work for him. Zabuza was the first to attack the group of ninja that were protecting the man he’d been hired to kill, eventually being overcome by the infamous Sharingan Kakashi, and his band of children. Just when Kakashi was about to kill him, Zabuza was ‘killed’ by a hunter-nin, a young boy in an ANBU* mask.
Yes, the boy was Haku and he had rescued Zabuza. It was soon apparent that Zabuza and Haku had planned the whole attack in order to find a way to counter Kakashi’s main technique, the Sharingan. Haku smiled and said that he had indeed found a way to counter the attacks of the Sharingan. The two retired for a short while, to heal, and plan.
During this respite, Haku met Naruto in the forest while out gathering medicinal herbs. He spoke to Naruto, asking him why he had become a ninja, and if he had someone he wanted to protect. He spoke the words that would eventually help shape Naruto’s entire outlook on the Ninja way of life. “A ninja is always strongest when protecting something important.”
Zabuza and Haku appeared to attack Kakashi’s Team 7 with the intention of killing them to get to the man they were protecting. Haku was to attack Sasuke while Zabuza, armed with Haku’s advice, was to attack Kakashi. Sasuke did quite well, until Haku began to unleash his Kekkei Genkai. In a short period of time, Haku had trapped Sasuke. This was when Naruto appeared and … completely ruined Sasuke’s original plans by becoming trapped as well. Haku spoke to the two trapped boys, telling that he didn’t wish to fight and that they should just give up even while they attacked and he countered.
Then, Sasuke, in an attempt to protect Naruto, is injured badly by Haku. As he falls, apparently dead, Naruto goes berserk, unleashing the Kyubi’s* power and nearly killing Haku. It isn’t until Naruto recognizes Haku as the same boy he had met in the forest that he stops. Haku smiled at Naruto and asks him why he didn’t kill him. Naruto can’t bring himself to do so and he says as much. Haku looks down at the younger boy and his smile is very sad. He tells Naruto of his past, and of his subsequent “rescue” by Zabuza-san. He tells Naruto that he is simply Zabuza-san’s tool, and now he is useless. He couldn’t fulfill his wish and now all he wants is to die. Naruto, taking pity on Haku, readies himself to kill Haku, but Haku stops him. He apologizes, saying there is something else he must do and then disappears.
While Haku had been fighting Naruto and Sasuke, Zabuza had been fighting Kakashi and was losing badly by this time. Kakashi was preparing his final attack and, just as he was about to kill Zabuza, there stood Haku, Kakashi’s hand through his chest, his blood everywhere. Without doing much more than cough up more blood, Haku fixed his dying stare on Kakashi and gripped his arm with what remained of his strength, telling Zabuza to kill Kakashi. Those were his last moments and, before either Kakashi or Zabuza could move, Haku was dead, Kakashi still locked in his tight grip.

Haku is bright, friendly and straightforward when dealing with others. He will not hesitate to make his point known, but he will happily listen to others as well. His loyalty is absolute once he gives it, but it can be difficult to earn.

Haku is an optimist. He likes to try and find the good in everyone around him, or, failing that, someway he can at least relate to them. He is also something of a sadist and likes to tease others. Never maliciously, but with little things. He does like to throw people off when it comes to his actual gender. He is a boy, thank you very much.

He has, however, been raised to think of himself as a tool, specifically Zabuza-san’s tool, but the fact remains that he has little to no regard for himself. He was born a thing that even his father hated so much that he tried to kill him, and then, during his stay on the streets, he became invisible, nothing but a beggar. When the one person who needed him came along, he was only useful as a tool, a weapon, so that is how Haku sees himself.