Pelias is Jason's power-hungry uncle who kicks Aeson, Jason's father, off the throne of Iolcus. Pelias does a fantastic job of playing the role of the Total Jerk, but if you take a look at his childhood you can get a peek into where his jerkiness (jerkitude?) might come from.
Along with his brother, Neleus, Pelias was abandoned by his mother, Tyro, on a mountain. See, Tyro had been tricked into conceiving the boys by Poseidon, who had disguised himself as her lover, the river god Enipeus. Later on, Tyro married Kretheus, King of Iolcus, with whom she had Aeson.
Raised by shepherds, Pelius grew up totally dissatisfied. Shepherding? Sounds like a bogus life for a son of Poseidon, doesn't it? The bitter young man was determined to be a king. So, as soon as he could, he went and seized the throne of his half-brother, Aeson.
Along the way, Pelias killed Tyro's stepmother, Sidera, who at one point had been mean to Tyro (Pelias' mother). Of course, he made a really big goof when he did this, because he killed Sidera on the altar of Hera. The queen of the gods was not what you'd call a forgiving lady, and she made it her mission to muck around with all his plans. Hera's hatred of Pelias is a big part of the reason she helped out Jason.
Not Without Precedent
Pelias goes down in a long list of wicked kings whose main purpose seems to be to make some young handsome hero's life miserable. He reminds us a lot of King Eurystheus, the guy who assigns Heracles his Twelve Labors. However, just like with Heracles, Jason's quest ends up making him famous. In the end, Pelias is brought down by his own devices. Sorry bud!