There are 4 rumors on how to unlock the Yellow Knight: * Beat Insane Mode with every character (Excluding DLC characters) * At the Character Selection screen, press the following buttons: Y,LB,RT,LB.LB,LT,Y,X,X, then select Orange Knight. * Beat the game with Hatty Hattington. * Reach level 70 on Insane Mode at Full Moon and beat it with Pink Knight or Civilian. Obviously, these theories are false.