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If you want to counter Prankster Plankton, then he's the best option. The only downside is, you have to make sure Mr. Krabs is holding a Krabby Patty, to lock Plankton into attacking moves only.
Outside of being a Plankton Counter, he doesn't really see that much action, being that he's and outclassed Defensive Water-Type.
However, if we're talking Doubles, then he does great when paired with Huge Power Pearl, and even when teamed up with The Appetizer, which is one of his better counters in Singles.