The Mule

A MULE, frolicsome from lack of work and from too much corn, galloped about in a very extravagant manner, and said to himself: "My father surely was a high-mettled racer, and I am his own child in speed and spirit." On the next day, being driven a long journey, and feeling very wearied, he exclaimed in a disconsolate tone: "I must have made a mistake; my father, after all, could have been only an ass."

He has the power to manipulate minds to his will, and by doing so looks to conquer the universe, and as we learn later, included in this is destroying the Second Foundation, as he sees them as his only real threat. The only thing stopping him, is that he doesn't know where they are. Basically, he wants to do this because he is a social misfit, and feels very rejected, so all this is his revenge.

Eventually, he is defeated by the First Speaker of the Second Foundation, when The Mule is outwitted, and his guard goes down for just a moment, long enough for the First Speaker to manipulate The Mule's mind, and cause him to lose all his ambition of conquering the galaxy, and to resign to live out the rest of his short life still as a ruler, but without bothering the Second Foundation, and letting them repair the damage that he has done.

In later books, we learn of The Mule's possible origins from the planet Gaia.