Papyrus actually loves puns, can do resets like Flowey and the player, and...  Knows more than even Sans or Gaster do.  He pretends to act naive and childish, but he's seen the horrors of no man's nightmare.  He can break the laws of physics and has extreme control over the game, able to higher or lower anyone's defense or attack at will and refuses to kill the player or user.  He doesn't know how he got into the underground because Sans only makes up storys.  Papyrus sees right through any lie.