Haku was a villager from the Land of Water, and a descendant of the Yuki clan. He later became a shinobi after meeting Zabuza Momochi whom he partnered with, ultimately becoming a mercenary. He was the little boy that Zabuza called his "tool," but who at the end treated like his little brother.
Haku was born into a peaceful farming family. One day, however, he accidentally realized that he could manipulate water, and showed his mother. His mother, knowing that the villagers hated those with these special abilities (kekkei genkai) bc they blamed the wielders with bringing war, chastised him and told him to never do that again. Haku's father, however, saw this display and decided to sacrifice his family for the sake of the village. He attacked his wife and child with a mob, but before he could kill Haku, Haku released ice spikes that killed his father and the rest of the village mob. Haku, however, was left an orphan.
One day, while quietly freezing on the streets, Zabuza ran into him. Something attracted Zabuza to the boy (Haku said they had the same eyes), and Zabuza asked Haku to become his "tool." Eventually Haku learned from Zabuza and became his loyal servant.
The first time we really see Haku is when he pretends to be a hunter-nin and takes Zabuza away right before Kakashi can finish him off. We then see Haku meeting Naruto while Zabuza recovers from his fight with Kakashi. Haku is the one who teaches Naruto that to become truly strong, one must fight to protect someone important. This scene was actually really weird as Naruto seems to develop a crush on Haku, who was a boy but "cuter than Sakura" (haha).
In the next and final time Haku appears, he takes on Naruto and Sasuke. While Sasuke re-awakens his Sharingan to take on Haku, Naruto struggles to keep up. So as Haku seemingly launches an attack at Naruto, Sasuke launches himself to protect Naruto taking the blunt of Haku's attack. Naruto enraged by Sasuke's "death," taps into the Nine Tails' power and overpowers Haku. It is at this point that Haku's face is revealed, and Naruto upon seeing it is the boy who was "cuter than Sakura," stops and regains his composure. Haku, however, asks him to finish him off, but as Naruto is about to do so, Haku notices Zabuza is about to be killed so he rushes over and takes Kakashi's attack, thus killing himself.
Haku to me is the perfect example of what happens to good people who just want a reason to live. They follow the wrong ideals and become "tools" for an evil cause when they, themselves, were just loyal good people. The deaths of Haku and Zabuza was truly a very emotional moment, or was that just me? haha