Haku is young teenage boy (at least that's what he looks like) who works for Yubaba. Lin tells Sen that Haku is one of Yubaba's henchmen that carries out some of her wicked orders (which is somewhat true to the viewer's knowledge).
Kohaku is actually a twelve-year-old river spirit who's river, which was also his home, was drained and lost to him. Yubaba stole his true name from him and left him with the name Haku, and so he was forced to work for her. Haku meets Chihiro and helps her save her parents and get home. Secretly, the young spirit is in love with Chihiro, but he's too shy to tell her. Little does he know, Chihiro loves him, too, but is also too shy to tell him face-to-face.