Plankton

Plankton is the main antagonist in Mermaid Man Forever! by Lovingtoomuchiowa.

Biography
According to the citizens of Bikini Bottom, Plankton was an evil terrorist, who struck fear into the hearts of everyone. There is no evidence for this, so you'll have to take their word for it.

Personality
Plankton is very evil. There is nothing that he won't do to in order to steal the Krabby Patty, the Krappy Patty, the Krabpy Patii, and the Krabbai Patty. He also likes to sing totally original, not at all ripped off songs.

Powers
Plankton can allow himself to be eaten by others, and then takes over their bodies. Although it is unknown how he obtained this power, one can assume that he is just so evil, that he spontaneously gets new powers.

Trivia

 * Despite being the main antagonist in the story, he disappears halfway through