Joseph Gribble

From King of the Hill Wiki

Joseph John Gribble is the 'son' of Dale Gribble and the best friend of Bobby Hill. Although he doesn't know it, his biological father is John Redcorn, which is the result of his mother, Nancy Gribble, cheating on his father while he went to hunt for aliens. He is among the few characters to undergo a change in appearance in Season 5 he was among the first middle school students at Tom Landry Middle School to hit puberty causing a change in voice and appearance, in the later episodes he is very awkward, geeky, and not very bright but is a great football player.

Joseph was voiced by Brittany Murphy from seasons 1-4, and Breckin Meyer from season 5 onward.