Character Analysis
We Smell a Rat
The most subtle thing about Peter Pettigrew is his name. We're surprised he isn't named Sketchy McTraitorface, in all honesty. The true betrayer of Harry's parents is in disguise as a rat, traditionally not a very honorable animal. (No offense to the adorable rat from Ratatouille.)
And when Pettigrew returns to human form, he still looks like a rat. (No offense to actor Timothy Spall.) Everything he does just seems oily and greasy. He even tries the tactic of comparing Harry to his father.
PETTIGREW: You look so much like your father. Like James.
It works when Lupin does it, but not when Pettigrew does. Maybe because his sniveling nature. Or because he uses Harry's dad's first name, as if Harry forgot it.
Pettigrew serves two purposes in Azkaban. One, he escapes, which makes us look forward to his capture in future books. Two, and more importantly, he allows Sirius to demonstrate his true nature. Sirius declares that Pettigrew is the true betrayer:
PETTIGREW: I didn't mean to! The Dark Lord. You have no idea the weapons he possesses! Ask yourself, Sirius! What would you have done?
SIRIUS: Died, rather than betray my friends!
And right there is the moment when Harry accepts Sirius as his godfather. Because Harry would do the same for his friends, too. He's no rat.