Yu-Bird is a supporting antagonist in Spirited Away. She is a seemingly androgynous, elderly vulture-like harpy creature and appears to be one of Yubaba's servants and her spy.
She made her first appearance when she was seen flying above The Bathhouse soon after the very first nightfall in the movie. She appears to show some characteristics of Yubaba; their faces are similar in shape and features.
Later in the film, Yu-Bird was transformed into a very small creature, resembling a fly, after Zeniba made her first appearance to Chihiro, Boh, and Haku. Boh takes a liking to Yu-Bird after their transformation; they can both be seen throughout the later parts of the film riding on Chihiro's shoulder and playing together.
It is not made apparent in the movie if they had a positive relationship prior to being transformed or not.