He had become Yubaba's apprentice to steal her magic, and he also stole that of her twin sister Zeniba.
In the original Japanese version, he was voiced by Miyu Irino, while in the English version, he was voiced by Jason Marsden.
Haku appears to be around 12 years of age in physical age. He has straight dark green hair in a bob haircut and slanted dark green eyes when he is human. Haku wears blue short hakama, white kariginu with blue kimono underneath and light brown sandals. When he transforms into his dragon form, he has a mint green mane and a white scaly body.
He is kind-hearted, but is the apprentice of the bath house's ruler, Yubaba. He has known Chihiro since she was little. In fact, what eventually proves it was a memory she has in her childhood was that when she falls into a river and loses a little pink shoe, and the river, known as the "Kohaku River", carries her to shore. She then decides that his real name is "Kohaku River", which meant he was the river spirit.
Haku first appears when Chihiro is on the bridge of the bath house. At this, he quickly yells to her to "Get out!" and get across the river before nightfall. But, when Chihiro fails to do this, she runs off. Then, in a panic, he goes in search for her and tells her that she is becoming invisible, he then gives her something to eat so she won't disappear. Haku then unbinds her when her legs become stuck and takes her to the bath house. Then, he helps her cross the bridge and in the garden outback, he tells her that she needs to get a job to survive from Kamaji the boiler man.
After Chihiro succeeds in getting a job, Haku then hands her off to Lin. The next morning, he takes her to her parents who have turned into pigs. Haku then warns Chihiro that if she forgets her real name like he forgot his, then she will be trapped in the spirit world forever. Then, Haku leaves and comes back days later gravely injured by paper birds and blood dripping from his body. Now, Haku ends up in Yubaba's office bleeding and close to death. Having Chihiro on his back, they end up falling down a mine shaft and into the boiler room with Kamaji.
Then, while Chihiro is off returning the seal that Haku allegedly stole, he then tells Yubaba that he will get her son back and flies off to Zeniba's (Yubaba's twin sister). Flying in the night with Chihiro on his back, Chihiro tells him that when she was little she fell into a river to retrieve a little pink shoe and that she thought she would drown, but, he saved her. She then tells him the river's name was the Kohaku river and thus revealing his real name. Both of them return to the bath house, and they say their good-byes at the river which has now been filled in.
Haku promises, however, that he and Chihiro would one day meet again as he lets go of her hand and watches her leave to go find her parents.
- Since the film's popularity, Haku had become one of the most ongoing anime heroes ever, along with Totoro.