Haku

Haku is the apprentice of Zabuza in Naruto, serving as the titular protagonist's first major opponent.

He acts as the secondary antagonist in the Land of Waves arc.

Background
Haku is of the Yuki Clan who were forced into hiding due to their fellow kinsmen, having suffered from a civil war, growing to despise any ninja born with a Kekkai Genkai, a form of ninjutsu genetically inherited. Haku's mother, a member of the Yuki clan, married a farmer and hid her heritage in order to live a peaceful life. However, when Haku's father learned of this and gathered a mob to reluctantly kill his family. Forced to watch his mother die, Haku retaliated in self-defense using his Ice Style to kill his father and the mob. Orphaned, Haku was forced to live in the streets and fight to survive until he met a kindred spirit in Zabuza Momochi and bonded. Willingly agreeing to be his tool, Haku trained under Zabuza in the ways of a ninja before and after leaving the Land of Water in exile as Rouge Ninja.

Part I
When Zabuza was hired by Gatō to kill the bridge builder Tazuna, Haku provided back up by posing as a maske Hunter Ninja to get Zabuza to safety after he is overwhelmed by Kakashi Hatake. While Zabuza is forced to recuperate in a week's day to finish the job, Haku meets Naruto Uzumaki as the two get into conversation about their purposes in life and those they care about. Naruto mistakes him for a lady which he clarifies when leaving.

A week later, as Zabuza battles Kakashi and wearing his mask, Haku engages Sasuke Uchiha in battle and overwhelms him with Naruto's appearance not tipping the scales. However, when Sasuke appeared to have died protecting Naruto from a "fatal" blow, revealed to have actually been knocked out, Naruto defeats Haku by subconsciously accessing the power of Kurama. Recognizing Haku upon shattering his mask, Naruto regained his senses and refuses to kill Haku even when he asked it as he felt he failed Zabuza.

However, seeing Zabuza about to be killed by Kakashi's Lightning Blade, Haku takes the fatal blow instead. Before his own death, Zabuza attempted that he did see Haku as more than a pawn while asking Kakashi to place him beside Haku in death.

Part II
During the Fourth Great Ninja War, Haku was reanimated by Kabuto Yakushi to fight against the Allied Shinobi Forces. Confronting Kakashi alongside Zabuza, Haku is glad to hear that Naruto has advanced since he last saw him. But as Kabuto purges his mind to make him an mindless monster, Haku's final moments of free will was gaining the knowledge from Kakashi that Zabuza did see him as a dear friend and not a tool. Haku is eventually immobilized by Ensui Nara and later bound and sealed by Maki until Kabuto's Reanimation Jutsu is negated so Haku's soul can return to the afterlife.