The Mule is a fictional character from Isaac Asimov Foundation Series.
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The Mule is a powerful telepathic mutant that creates the biggest threat to derailing of Seldon Plan because his telepathy gives him the capacity to disrupt Seldon's plan by invalidating Seldon's assumption that human emotional responses to stimuli will remain the same.
Motivated by awareness of his abnormal appearance and hatred against the galaxy, the Mule used his extensive powers to rise to power and conquer even the Foundation. The Mule uses his telepathic ability to carve his own empire out of nearby territory, sweep aside the remnants of the Galactic Empire centered around Neo-Trantor. The Mule based his rule on the planet Kalgan, which he obtained by mentally converting the warlord of Kalgan. After conquering the First 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.
Following his conquest of the Foundation, the Mule attempted to locate and destroy the Second Foundation. In the end the First Speaker of the Second Foundation mentally altered him, whereupon he ended his quest and governed as a benevolent dictator until he died a premature death resulting from his mutation.
The Mule is a central character in both Foundation and Empire and Second Foundation. In Foundation's Edge we are told that he originally came from planet Gaia but was regarded there as a criminal.
His nickname
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.
Physical appearance
The Mule was a physical freak. His nose was incredibly long, sticking out over three inches from his face. The flesh of the rest of the Mule's face seems stretched across his skull. The Mule's body was spindly and gangly: he weighed only 100 to 120 pounds (45 to 54 kg), stretched out across a 5'8" 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.