Pig pen is never a main character - you hardly ever hear his
   name (does anybody know his name?), he just does what he does
   and is pretty much known only by the dirt or mess he leaves
   around him and yet he is the only peanuts character that seems
   to have no major character flaws, no psychological conundrums,
   not a lesbian, a sidekick, a know-it-all, a loser, an obsessive
   pianist, an annoying little sister. I'd have gone with the
   kite-eating tree, but it's not as recognizable. "