In Studio Ghibli’s Spirited Away, Yubaba gives Chihiro the name Sen after taking her name.
Yubaba is the witch who runs the bathhouse where Chihiro works after her parents are transformed into pigs. In the employment contract, Yubaba removes most of Chihiro’s name and leaves her with the single-character name Sen. This loss of identity is a central part of the film’s story.
Haku warns Chihiro to remember her real name, because forgetting it could leave her trapped in the spirit world. Haku himself has also forgotten his identity, though he later remembers that he is the spirit of the Kohaku River.
Sen is not Chihiro’s nickname from before the bathhouse visit. It is the name imposed on her by Yubaba, and the film uses the distinction to explore memory, identity, and growing independence.