Nemesis

Nemesis are things that are kinda like Newton's (number) law. It says "For every action there is an equal and opposite reaction" except without the EQUAL part.

Examples

 * Phineas and Ferb: Dr. Heinz Doofenshmirtz vs. Perry the Platypus


 * SpongeBob: The Krusty Krab crew vs. Plankton


 * iCarly: The iCarly cast vs. Nevel/Neville/However you spell it

et cetera

Usually, one is good and one is evil (Perry, the Krusty Krab crew, and the iCarly cast are good, and the others are evil)