Jason

Jason is one of the more despicable human beings I've ever encountered, literature or otherwise.  He seems to take a perverse joy in being difficult and rude at every opportunity.  I think there's some resentment going on as well.  The family sold some of their property to send Quentin off to Harvard, which ended up being a bust with the whole suicide incident.  Caroline claims Jason is her favorite child, yet the family didn't go to any great lengths to further his life and career like they did with Quentin.  Sure, Jason says he wouldn't want to go anyway, but come on, even if the experience at school itself was awful, Jason's so concerned about image that having the bragging rights of being a Harvard graduate would be enough for him.  

He hates Quentin, both of them, one for getting what he never did I think and the other for existing quite simply, and the money that is required to care for her, which is ironic because Jason's been stealing more than enough money through Caddy's checks to make up for that deficit.  But of course he's also been gambling in the stock market, complaining all the while.  That's another thing, despite all his big words, Jason is not a man of action.  He threatens to beat Quentin, but never does, likewise he moans about the money he pays the telegraph office for market advice, writing it off as useless and a waste of money, but continues to pay them.  And then there's also his horrible work ethic and behavior-he clearly hates his job, but he won't quit, he just keeps trying to goad Earl into firing him, so far to no avail.

All in all, Jason's a grump, but lacks a spine.