Bob the Boil is the large boil on Elmer's face.
As Elmer became a more important character, it was revealed that his boil can talk and has a mind of his own. The boil's name is Bob, and his biggest wish is to take over the world. He has control over Elmer, which he also considers to be his best friend. Bob does not seem to like Timmy Turner, as when Timmy was able to read minds, he could hear Bob telling Elmer to get rid of Timmy.