Bruno Jenkins is the tritagonist in the book and film The Witches. He was a boy before he was turned into a mouse.
Bruno was a rich boy who lived with his parents and went on vacation to a hotel in England. There, he met a boy named Luke Eveshim. They had a bad start, Bruno boasting about how his family was richer than Luke's and teasing him. One such confrontation ended with Luke attacking Bruno, who ran away and did not bother Luke again.
He was then confronted by the Grand High Witch in disguise, who gave him a chocolate bar that was covered in a potion. He ate it of course. The Grand High Witch revealed the next day that he would turn into a mouse, and he came to the meeting of the witches of England, not knowing of this, and ended up turning into a mouse and running away. However, he found some crumbs and ate them, so focused on eating that he did not even know he was a mouse.
Luke was turned into a mouse as well and came to him and told him that they were both mice. They went to Luke's grandmother Helga, who knew a lot about witches, and she and Luke came up with a plan. Helga also came to Bruno's parents, but Bruno's mother was scared of mice and freaked out when she saw Bruno, not knowing it was her son. Bruno's father later confronted them when Bruno did not show up, and Bruno talked to his father. At that moment, the witches all turned into mice because Luke put the potion in their soup.
After the witches are killed, Bruno is returned to his parents, still a mouse. In the film, there is a different version: he is turned back into a human by a reformed witch, Susan Irvine.