Kernac is the adversary of the Letter People episode "Meet Mister K." He is a wizard who has kidnapped the King of Kabob and hidden the magic key somewhere.

That somewhere is in a kit that contains four keys total, including the right one (which has a lowercase K, while the other three each have a capital K).

In a fantasy Mister K has, Mister K, Miss I, and Mister T play the catching game with Kernac to trick him into revealing that the key is in a kit.

Back to real life, Mister K tries the key with the lowercase K and frees the King, much to the dismay of Kernac, though it is shown that he has stolen the king's kazoo, preventing him from having a kazoo concert at his kindergarten.