
(and also to distract myself from the fact that I picked way too broad a topic for my paper and probably need to find a new one)
He's quite clever, when it comes to something he's interested in. Unfortunately, he's also thoughtless and lazy when it comes to things he doesn't care about, or things he cares about but doesn't know how to go about framing in his head. If you can talk him into thinking about things, though, he can do it quite well. He just doesn't always like to, which is why he avoids doing so.
This is why he's only average in most of his classes; they're not what he cares about, and he only needs to maintain a minimal level in order to do the things he does care about (quidditch, chess, strategy, playing around with friends).
He mostly reads chess books and things for schoolwork, but he quite enjoys a certain kind of history book. You know, the ones about generals and wars and rulers and politics, that are far enough removed from daily life that they can be thought of as abstractions and not applied to the real world at all if you don't need them to. (I also like this kind of book, so that's how I know.)
He thinks most people are pretty decent, given the right circumstances and plenty of time. Maybe he fights with Malfoy, and maybe they don't particularly like each other, but they could probably get along if they had something to talk about and the issues of family politics were removed. Maybe he doesn't like Professor Snape, but he's probably an okay bloke if you like Potions.
He is easily cruel to those who've hurt the people he counts as his. (This doesn't mean he hates them.)
He is easily envious and jealous. He wants to be special, he wants to stand out from his brothers but still fit in with his family. He wants to have money for the little things people like Malfoy or Harry don't think about, new clothes and toys and schoolbooks. (He understands Hermione's fascination with marked-up books, but he'd rather have something new and shiny.) Not even big things, like brooms -- just the little things of everyday life. If he lived in the Muggle world, he'd probably love buildings made of steel and glass.
One of the things he's always loved about Hogwarts is that there's always somewhere you can go off to and be alone when you need it.