Bruno Jenkins is the dictionary definition of an oaf. He's big, he's greedy, he's slow-witted, he's just... oafy. (We made that word up, but we're allowed to after all the made-up words in The Witches.) He's always eating, he brags about his dad's wealth, and he even kills ants with a magnifying glass.
Bruno might not be our favorite character, but he's definitely an important one. He's the only character in the book, other than our narrator, to experience the human-to-mouse transformation and live to tell the tale. Because he has such a particular personality – oafy, in case you've forgotten – he's the perfect figure to show that the witches' magic formula doesn't change a person on the inside, just the outside. After the Grand High Witch turns Bruno into a mouse, he's still the same old Bruno – so busy eating that he doesn't even notice he's a mouse (13.25), so busy eating that he can't convince his parents that he's a mouse (17.2), so busy eating that he doesn't even take part in the anti-witch planning (18.2). Do you notice a pattern?
Bruno also serves as a foil to our narrator (see the section on "Character Roles" for more on that), showing us how a typical boy his age might act. Because of Bruno, we're able to see more clearly how awesome, humble, and mature our narrator really is.