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House: Gryffindor, 3rd Year
Ron is in this novel a fair amount. During the shunning of Hermione, we pretty much just get scenes with him and Harry. But it seems like Ron isn't actually doing very much. Compared to Harry and Hermione, in Book 3 he doesn't undergo an extreme emotional experience or mature in a notable way; he doesn't have a big heroic moment. Instead, for most of the book he acts like a kid – a petulant bratty kid. The fights he gets into with Hermione are very childish, and there's an element of childish cruelty in them – just recall that scene in which he rubs it in about his rat and makes Hermione run off crying.
And there's a history of this sort of behavior too – if this novel is largely about confronting darkness head-on, including darkness and negative traits within oneself, then we definitely get to tackle Ron's negative traits throughout this one (while never losing sight of his positive, pre-established traits as Harry's friend).
The Dark Side
Ron spends much of the novel acting jealous, letting his temper get the best of him, and being immature. Let's just look at a few examples among many:
He dropped his voice [...] and told them all about the Marauder's Map.
"How come Fred and George never gave it to me!" said Ron, outraged. "I'm their brother!" (10.3.77-78)
Ron became an instant celebrity. For the first time in his life, people were paying more attention to him than to Harry, and it was clear that Ron was rather enjoying the experience. (14.1.7)
"Can't you give her a break?" Harry asked Ron quietly.
"No," said Ron flatly. "If she just acted like she was sorry – but she'll never admit she's wrong, Hermione." (13.3.11-12)
Yeah, this isn't Ron at his finest. His fame monster antics in particular are a bit disturbing, showing a side of his personality that isn't all that great. And his stubborn refusal to forgive Hermione reveals a sort of childishness.
A lot of this is probably an unintended consequence of his family life. Ron constantly struggles to be an individual in his large family and he's constantly overshadowed by everyone else around him. Percy is a super smart head boy; the twins are popular pranksters and Quidditch stars; Ginny is the only girl, so she stands out without trying. And Ron is just a nice kid with a genius BFF and a super famous hero BFF. We'd probably be whiny and moody too if everyone around us was a superstar and we were the perennial sidekick.
The Light Side
But Ron isn't a jerk (all the time, at least). He's a really great friend, especially to Harry. His relationship with Hermione is marked by so much tension and competition and stubbornness that it's hard to sort out exactly in this book. Which is fitting because Ron's own feelings about Hermione seem to veer all over the place – one second he's worried about her crazy schedule, the next he's making deliberately hurtful comments to her about her ugly cat. It's a bit nuts.
But back to Ron and Harry. Ron provides Harry with a much-needed pseudo-family and his sense of humor is very welcome. Ron gets in more one-liners than anyone else in the book probably. So we can chalk up a lot of Ron's bad behavior to growing pains (though there are some darker aspects to his personality that might cause problems in the future). We do have one big question, though....
A Boy and His Rat
The main thing that Ron really does in this story is obsess over his lame rat and serve as a sort of conduit for the events that unfold – it's his picture in the paper, accompanied by his rat, that leads to the domino effect of Sirius escaping prison. So, really: what's the deal with Ron's concern and even obsession over his rat? Is it just an excuse to argue with Hermione? A way to draw attention to himself? Genuine concern for small animals? We vote for a combo of those things – that rat was one of the few things that was actually Ron's (albeit a hand-me-down from Percy). And when you have very little to call your own, you get rather possessive of what you actually do have.