Garion was set up by Polgara and Belgarath as a puppet king.
Bear with me. Garion is raised by Polgara on Faldor's farm, and spends most of his childhood doing menial tasks. Now, while this does make sure he never develops a sense of superiority, and it makes him a very sensible person- both valued virtues in a king-, he was also continually kept in the dark by Polgara on nearly every subject, and this makes him totally dependent on her, because as a very sensible person, Garion isn't going to try tackle a problem he has no idea how to handle by himself, he'll just call his Aunt Pol. This gives Polgara the ability to continually interfere in the workings of the Western kingdoms so she can make sure that the events of the Prophecy happen. Once Garion grows up and gets the hang of the job, he isn't dependent on Polgara any more- but she raised him, so mostly, what he does will be what she wants, especially since he loves her and admires her and has put her on a pedestal. Belgarath, in the meantime, is there to serve as another figure for Garion to depend on, while serving the same goals as Polgara. While Polgara and Belgarath do care for him, they care for the Prophecy even more.
The majority of the characters in the Belgariad/Malloreon were set up as deconstructions of the usual fantasy stock characters.
Take Garion, your traditional 'chosen one who is revealed to be so after labouring in obscurity for x years'. He's a badass, a good king and all- but he's also extremely dependent on the people around him, insecure, headstrong, and rarely thinks things through (remember the potential ice age-causing storm in Guardians of the West?). Ce'Nedra, the 'feisty princess who marries the chosen one' character, is a hypocritical bitch who spends most of her time in books as The Load and often delays and slows the main cast down. Polgara and Belgarath, the 'wizard advisors who help the main character and do awesome things a lot' characters, are both extremely manipulative. Belgarath's a drunken, irresponsible lecher and Polgara's a superior, condescending bitch. And that's not even all of them...
- Much of Belgarath's irresponsibility and other flaws are either 1.) caused by his eternal grief over the loss of his wife and fellow disciples, or 2.) an act or exaggerated by him so that people won't think he's Belgarath, giving him a free hand to manipulate people and events to further the Prophecy.. In my opinion, the deconstruction comes from just how manipulative someone would have to be to keep up said facade and basically engineer human history to fulfill an incredibly vague prophecy. Silk is an incredibly badass, cunning, snarky superspy; he also has serious family issues (which are implied to be what led to his becoming such a skilled spy, since it's an excuse to spend as much time away from home as possible) and a very casual disregard for crimes up to and including killing. Mandorallen (and indeed all Arends in general, and Mimbrates in particular) is the typical invincible White Knight common to medieval epics, but shows just how stupid one would have to be to have no fear at all, as well as how totally lost they would be when they actually felt fear. Durnik is just your average salt-of-the-earth everyman dragged along on an epic quest, but starts hardening due to the perils they face, several times suggesting or committing very callous acts (asphyxiating Murgos with smog, drowning them with quicksand, etc.), and so on.