Bo is the son of Yubaba, the witch who operates the bathhouse in the film. He is a yokai baby, a supernatural being from Japanese folklore, and the nephew of Yubaba's twin sister, Zeniba. Despite being an infant, Bo is physically enormous, about twice the size of his mother, with a very chubby and overweight appearance, and he wears only a red bib with his name written on it. He has been raised in an extremely sheltered and overprotective environment, having spent his entire life in the nursery adjacent to Yubaba's office, never venturing outside because his mother taught him that doing so would make him sick.

Initially, Bo has a thoroughly selfish, spoiled, and temperamental personality, the product of being indulged with all the attention, gifts, and food he desired. He is prone to violent tantrums when displeased or when his demands are not met, using his immense, superhuman strength to smash through doors, and his crying causes the entire area to tremble. He also has a developed fear of germs, known as mysophobia. His main motivation at the beginning of the story is simply to have his desires for play and attention satisfied immediately, as shown when he forces Chihiro, who is working under the name Sen, to play with him by threatening to cry and call his mother.

His role in the story changes dramatically when Zeniba, disguised as a paper shikigami, transforms him into a small, plump mouse as punishment for interfering. In this new form, he inadvertently becomes Chihiro's companion on her journey to return a stolen magic seal to Zeniba. He is often carried by Yubaba's pet bird, who is simultaneously transformed into a small fly-like creature. This adventure marks the beginning of his development. Through accompanying Chihiro, Bo is exposed to the outside world for the first time and begins to mature significantly. He imitates Chihiro's actions, such as mimicking her when she is cleansed of a curse, which is an essential step in his coming-of-age process. He shows signs of becoming more self-sufficient, such as carrying the transformed bird when it grows tired, and by the end of his journey, he has learned to stand and walk firmly on his own, something that surprises his mother, suggesting he had never done so before.

His key relationship is with his mother, Yubaba, who spoils him greatly, but he also forms a strong bond with Chihiro. Through his experiences with her, his loyalty shifts. In a pivotal moment upon returning to the bathhouse, he directly admonishes Yubaba for being stingy and refusing to let Chihiro go, telling her that he will not like her anymore if she makes Chihiro cry. This demonstrates how his adventure and his friendship with Chihiro have transformed him from a purely selfish child into a courageous and loyal individual capable of standing up for others. Regarding notable abilities, Bo possesses immense physical strength far beyond his infant appearance, allowing him to break through doors with ease. After being transformed into a mouse, he retains his consciousness and, once Zeniba's magic wears off, he gains the ability to return to his original form at will, though he chooses to remain as a mouse until the film's climax.