It’s commonly accepted that the Weasleys are a big family – SEVEN kids. SEVEN. That’s just…so many – but I don’t think it’s ever really mentioned much when people talk about the Weasley kids and their personalities. The fact is, growing up in a big family is totally going to shape your personality – the same way being an only child will shape your personality. Ron in particular – as the only one we really get a deep look into, personality-wise – has a weird combination of cruelty and loyalty that, when looked at in the context of his home life, makes much more sense.
The first time I read Harry Potter, I didn’t find Ron to be mean, or cruel at all. It wasn’t till I re read the books a few times – and discussed some of the characters with my friends who are HP fans – that I recognized that he had the tendency to thoughtlessly fling insults at people. If I put on my armchair psychiatrist hat for a second – I’m totally qualified in the wizarding world – I’d have to say it’s a function of growing up in a large family, where love is so assured that flinging insults is a natural form of communication (not that I know anything about that). For proof, I give you Ron standing up to Snape for Hermione when Snape covers Defense Against the Dark Arts in Prisoner of Azkaban. At this point, I don’t think Ron had any feelings towards Hermione other than friendly – possibly even sisterly – affection, but when someone else insulated her (using the same insults he uses himself) he jumped to her defense. It reeked of “I can say that because she’s part of my family, you’re not a part of this you don’t get to say that”.
Which ties directly into the second piece of Ron’s personality that I think is directly tied to his large family – his intense loyalty. Particularly after the first half of Goblet of Fire (when he openly doubts Harry and for a time doesn’t even talk to him), Ron is always, always, always on Harry’s side. When Harry is going on a life-threatening trip to destroy the most evil wizard of all time, Ron is not about to stay home. Even when Ron doesn’t agree with Harry (ahem Malfoy ahem) he still at least attempts to support him – and defend him against Hermione. It’s very clear that Ron sees Harry – and later, Hermione – as a part of his family, and is as intensely loyal to them as he is to his brothers.