That first chapter tells us quite a bit about Wormtail's character, despite being told from the PoV of a total stranger. What we saw in PoA was a sycophant and a slimy, cowardly character that even disgusted Hermione, who generally is quite willing to see the good in people (more so than her friends, anyway). This chapter nuances that a bit, I think. He's still a sycophant, and still afraid - has every reason to be - but we learn that he has displayed some courage, competence and clever thinking on his feet in the months gone by, even if it's for the wrong side, and that he feels a certain obligation towards Harry. Rowling depicts Peter Pettigrew perhaps more negatively - with less respect - than any other character in the series with the possible exception of Vernon Dursley, but it's not all bad. However, he did play a pivotal role in helping Voldemort get back, with all the consequences that will have in later books. What he had said already in PoA - that whatever his original motives for going to Voldemort, pretty soon it was just fear that kept him in his service - seems to apply still, but that's hard to reconcile with the way he helps Voldemort to get back on his feet. One assumes he was desperate for someone to protect him, someone to follow, and so went looking for Voldemort. Sort of like an abused spouse returning to his or her tormentor, except that the tormentor is also a mass murderer who is allowed to continue thanks to that. Dark stuff, that, if you think about it.

Dudley Dursley, always a mere caricature until this point, is finally beginning to become a more serious character - an evolution that will culminate in book seven. His parents are finally trying to remedy their massive failures in raising him, though reluctantly. He hasn't changed yet, really - the throwing the PlayStation out of the window fits with his character in the previous three books - but it seems safe to assume this diet is one of the factors that will see him change for the better in future books. As far as starting a lasting improvement in his parents' parenting goes, though, the diet is disappointing. In future books he'll grow as a character more in spite of his parents than thanks to them.