If you make Sen cry, I won't like you anymore!
- —Boh to Yubaba, regarding Chihiro.
Boy first appears as Chihiro Ogino tries to get a job at the bathhouse, while Chihiro argues with Yubaba Boh wakes up because of the commotion and starts crying and breaking through the door leading from his room to Yubaba's office. After Boh breaks through the door Yubaba gives Chihiro the job, as them arguing is what caused Boh to wake up and have a tantrum in the first place.
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.
- "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!"