He appears to Chihiro Ogino as a young boy around the age of twelve and has pale skin compared to Chihiro's lightly tanned tone. Haku has black shoulder-length hair with straight bangs and deep green eyes. His clothes allude to the traditional Japanese style of the Heian period, wearing what appears to be a white tunic style top which is secured with a purple sash. He is mostly seen with traditional Japanese sandals and is also around the height of 5'7".
In his dragon form, Haku resembles an Eastern dragon with a long, slender body and silvery scales. Haku's stomach scales are a pale brown, with sea green fur that acts as a flowing mane growing along his body, stopping in a tuft on his tail. He has a long snout and pointed horns atop his head that slant backwards positioned behind his ears. He also has long white whiskers and four wiry legs that resemble that of a rooster.
He first meets Chihiro on the bridge to Yubaba's Bathhouse, where he warns her to flee before sundown and tells her that he would try to buy her time for escape. Later, knowing that Chihiro had failed, Haku gives Chihiro food that prevents her fading into nonexistence. Dispelling a paralysis spell on her legs, Haku takes Chihiro to the bathhouse where he imprints instructions on finding Kamajii magically in Chihiro's mind. He then responds to the clamor made by the bathhouse workers.
Haku is then seen in the morning, quietly telling Chihiro that he would take her to go see her parents and comforts Chihiro when she cries. He returns to her the set of clothes she wore in the human world as well as the name card on her bouquet, reminding her to never forget her name. Haku then tells Chihiro that he forgot his name long ago, and that it was surprising even to him that he remembered Chihiro's name. Leaving Chihiro to return to the bathhouse, Haku is then seen flying off in his transformed dragon state into the distance.
Some time later, he steals a golden seal from Zeniba under Yubaba's orders and is put under the effects of Zeniba's death curse. Escaping with the gold seal but terribly injured, he responds to Chihiro's voice and admits himself to Yubaba's penthouse where Zeniba's shikigami projection reveals to Chihiro that Haku will die soon. Haku then uses his remaining strength to smash Zeniba's shikigami and falls into the disposal hole, falling into Kamaji's boiler room. Struggling to survive and bleeding profusely, Haku is given half of Chihiro's emetic dumpling, causing him to regurgitate Zeniba's golden seal as well as Yubaba's magically conjured worm. He is then given medicine by Kamaji and put to a makeshift bed to recuperate from his magically inflicted injuries.
Long after Chihiro leaves for Zeniba's place, Haku awakens and confronts a furiously angered Yubaba who realized that her son, Boh, had been replaced and taken away from her. Haku calmly states that he can bring Boh back to Yubaba under the condition that Yubaba leave Chihiro and her family alone from then on. He remains stoic when Yubaba tells him that what happens after he brings Chihiro back will not be pleasant. It is assumed that he takes off for Zeniba's place soon after this.
Haku appears later in front of Zeniba's cottage in his dragon form, bowing in apology to Zeniba for stealing her seal. He receives her forgiveness and leaves with her blessing, taking Chihiro, Boh and Yubaba's Bird back to the bathhouse in silence. Here Chihiro tells Haku that she recalled the past event when Haku saved her, revealing Haku's real name as Kohaku in the process. Happy that his curse is finally broken Haku transforms back into his human spirit form and thanks Chihiro. The two fly back hand in hand to the bathhouse
After Chihiro's winning of Yubaba's challenge, Haku is seen taking Chihiro back to the tunnel where her parents waited. Haku then sadly lets go of Chihiro hand, promising her that they will meet again someday and telling her to never look back, reassuring Chihiro of his safety now that his curse has been broken.
The audience is shown both a relieved and sad glint from his eyes when he lets go of Chihiro's hand in the final scene, both because he was glad that Chihiro can reunite with her parents and free herself from memories of the Spirit World. Haku's promise to her that he'll meet her again someday is questionable, for there is quite possibly no other way for the two to reunite until Chihiro's death or unlikely return to the Spirit World. This seems to be a clue from Hayao Miyazaki himself that will make us think about how their relationship will go on. However, Zeniba's quote, "memories are never forgotten, they are just difficult to recall" suggests that Chihiro will eventually remember her experiences at the bathhouse and her relationship with Haku. Regardless, the fate of their relationship is left to audience interpretation.
Initially shown to be most kindhearted, caring and compassionate, Haku serves as one of the factor's of Chihiro's massive growth of character throughout the film. He is considerate, crafting rice balls for Chihiro knowing that she would be hungry and going out of his way to show Chihiro her cursed parents. In Yubaba's presence however, Haku changes from a kindhearted person to a stoic, cold and strictly professional inidividual, treating everyone (including Chihiro) with sternness so as not to arouse suspicion in Yubaba.
- In the Japanese version of the film, Yubaba tells Chihiro that the price of failure in her challenge will lead to Chihiro having to kill and tear Haku to pieces herself. In the English adaptation Yubaba instead tells Chihiro that failure will lead to eternal imprisonment in the bathhouse.
- The name Haku means "master, lord, overseer", which may refer to his lofty position as Yubaba's respected apprentice as well as his power as 'god' of the Kohaku River.