The Mule is a fictional character from Isaac Asimov's Foundation Series. One of the greatest conquerors the galaxy has ever seen, he is a telepath who has the ability to manipulate human emotions. This gives him the capacity to disrupt Seldon's plan by invalidating Seldon's assumption that human emotional responses to stimuli will remain the same.
The Mule uses his ability to conquer the Foundation (and carve his own empire out of nearby territory), sweep aside the pathetic remants of the Galactic Empire centered around Neo-Trantor, and set up his own short-lived Galactic Empire — the "Union of Worlds", styling himself "First Citizen of the Union". The Mule based his rule on the planet Kalgan, which he obtained by mentally converting the warlord of Kalgan. After conquering the Foundation, he made Kalgan the capital of the Union. Leading up to, during, and for a good time after the Mule's conquest of the Foundation and its trade confederacy, no one ever actually saw the Mule or knew what he looked like.
The Mule established his empire incrementally, using past conquests to aid new ones: first by mentally converting a pirate band to his allegiance, then a whole planet, then the militarily powerful kingdom of Kalgan, then the Foundation (and imperial remnant). The Foundation, after the death of the Empire, was the sole supplier of nuclear weaponry in the galaxy, and using this asset the Mule began rapidly conquering surrounding territories, which all lacked nuclear power.
The Mule's conquest was amazingly fast: he defeated the Foundation and established the Union of Worlds after only 5 years of conquest. Then just as suddenly, and seemingly randomly, the Mule stopped his advance and settled into a 5 year period of consolidation. This was actually because the Mule feared the Second Foundation, rumored to be capable of defeating a mentalic individual like himself. However, the location of the Second Foundation was a total mystery. No one had ever heard anything other than the name, and besides rumors the popular sentiment was that there was no Second Foundation.
During this period of consolidation, the Mule launched repeated expeditions in search of the Second Foundation, which according to a hint by Hari Seldon existed "on the opposite side of the galaxy" from the First Foundation on Terminus.
His name is one he gives to himself, a reference to his genetic sterility. For this reason his empire does not last long after his death.
Physically, the Mule was a ridiculous freak. The Mule's nose was incredibly long, sticking out over 3 inches from his face. The flesh of the rest of the Mule's face seemed stretched across his skull. The Mule's body was spindly and gangly: he weighed only 100-120 pounds, stretched out across a 5'10 frame. Thus his joints, such as elbows, seemed to jut out from his diminutive musculature. The Mule's meagerness was glandular and could not be compensated for. Further, the Mule's limbs met at awkward angles, giving the general impression of a scarecrow assembled poorly. It was said that one could not look at the Mule without derision. The only aspect of the Mule which was not blatantly farcical were his eyes, which were described as deep brown and perpetually sad.
As a result of the Mule's freakish appearance, his childhood was one of torment, in which he was marginalized and alienated. The Mule became aware of his great mental powers in his twenties, and after becoming aware of his power, he developed a desire to compensate for his earlier life by taking revenge on all humanity in the entire galaxy.
The Mule is a central character in both Foundation and Empire and Second Foundation. In Foundation's Edge we are told that he was originally from planet Gaia but was regarded there as a criminal.
According to his autobiography In Memory Yet Green, Asimov modeled his physical appearance on his friend Leonard Meisel, a friend at the World War II-era Navy Yard in Philadelphia.