See, Rowling largely operates Harry’s generation in a clear system of parallels to the previous generation, Marauders and all. Harry is his father—Quidditch star, a little pig-headed sometimes, an excellent leader. Ron is Sirius Black—snarky and fun, loyal to a fault, mired in self-doubts. Hermione is Remus Lupin—book smart and meticulous, always level-headed, unfailingly perceptive. Ginny is Lily Evans—a firecracker, clever and kind, unwilling to take excuses. Draco Malfoy is Severus Snape—a natural foil to Harry, pretentious, possessed of the frailest ego and also deeper sense of right and wrong when it counts. And guess what? Neville Longbottom is Peter Pettigrew.
Neville is a perfect example of how one single ingredient in the recipe can either ruin your casserole (or stew, or treacle tart, whatever you like), or utterly perfect your whole dish. Neville is the tide-turner, the shiny hinge. And all because he happens to be in the same position as Wormtail… but makes all the hard choices that Pettigrew refused the first time around. Other characters are in similar positions, but none of them go so far as Neville. None of them prove that the shaping of destiny is all on the individual the way he does.