Boh: If you make Sen cry, I won't like you anymore!
- —Boh to Yubaba, regarding Chihiro.
He is portrayed as an extremely large and overfed sumo baby wearing a red bib that covers his front side, leaving the rest of his back exposed. Although a big sumo baby, Boh is able to talk, surprisingly, fluently for his apparent young age, and walk. His mother frequently spoils him, leaving him in a large cushioned room where nothing can harm him, and giving him whatever he wants on command. He is the only person in the world that Yubaba loves, apart from her love of gold.
He has remained within his nursery out of fear of germs, up until meeting Chihiro Ogino. During the events of the story, he is transformed by his aunt Zeniba into a small, grey mouse. While this must have been a traumatizing experience for such a seemingly young child, a great deal of character development in his personality can be observed.
Prior to meeting Chihiro, Boh is a selfish and spoiled child. He was used to getting all the attention, gifts and food he wanted from Yubaba. He also developed a fear of germs (mysophobia), demonstrated by his initial hindrance in leaving his nursery.
However, his personality quickly changes during his adventures with Chihiro. In his mouse form, he develops a strong affiliation with Yubaba's Bird, who was turned into a fly-like creature and would often carry him around when they were not riding on Chihiro's shoulder. By the end of these adventures, we see him literally standing up to his mother to help Chihiro.
It is also suggested that, prior to meeting Chihiro, Boh had not been seen walking or even standing up on his own. This is seen through Yubaba's surprise at seeing Boh standing after his return to her with Chihiro and Haku. It can be assumed that he simply became less lazy and more self-sufficient after his adventure.
He is two times bigger than his mother. His appearance and personality are inspired by the Kintarou of a Japanese tale.
In folklore, Kintarou wears a red Harakake, a bib, with his short name Kin on it. Boh is just a short name. His mother Yubaba, who is a witch, frequently spoils him. It is unknown who is Kintarou's father.
- On his bib, it says the kanji for his Japanese name (坊 Bou).
- In the English version of the film, his name was never said, only shown in the credits.
- He can't tell his mother from his aunt because they are identical twins.
- He was voiced by Tara Strong, the voice of Bubbles from Cartoon Network's The Powerpuff Girls.
- That Quite More than A Size Or A Sumo Baby
- "You'll get sick if you go outside. So stay here and play with me."
- "If you go, I'll cry. Then Mama will hear me and Mama will come in here and kill you."
- "Play with me, or I'll break your arm."
- "I'm not afraid of germs."
- "If you make Sen cry, I won't like you anymore!"
- "That's why I've never left this room."
- "Yes, you will."
- "If you don't play with me, I'll cry."
- "Right now, or I'll start crying!"