Peter has been unjustly labelled inept at wizardry by the fandom, and it's time someone spoke up on his behalf.
Peter is generally portrayed in fanfic (when he is portrayed at all, but that's a rant for another time) as being not only evil, but also stupid. Crabbe and Goyle look like veritable geniuses next to the blobby incompetence of Peter.
Yet this is fanon, not canon. Actually, Voldemort consistently trusts Peter to take care of him and to make potions restoring his health. Peter, mind you, not Snape.
And yet Snape is the canonical genius when it comes to potions.
So why would Voldemort use Peter as a maker of healing potions when--even in hiding--he could have potions made by Snape owled to him?
I'm guessing that Snape is the better at potions overall--but that Peter is the one who has a knack for healing. Snape doesn't. In Muggle terms, Snape is a chemist, but Peter is a physician.
I don't see Voldemort trusting his health or his life to an incompetent. He could surely terrify, torture or Imperio Peter into getting him into more able hands, if Peter's skill were inadequate to his needs. Voldemort is, after all, no respecter of persons.
"But," I can hear someone in the back protesting, "McGonagall said that Peter wasn't in the same class as James and Sirius, magically!"
I don't think he was.
I suspect that James and Sirius were naturals at magic--that they had brilliant, lightning-fast minds that grasped means and method light-years ahead of other students. Peter was slower to grasp things that they understood almost intuitively. He was a plodder, and plodders can be aggravating in classrooms, because they want to take their time, understand all the aspects of what they're doing, and get into the whys and the wherefores, if possible.
People tend to forget that slower doesn't mean less able. Some of the best thinkers in the world have been those who mulled over problems and concepts for years.
I think that the difference between James' and Sirius' magic and Peter's magic is illustrated by something that Gytha Ogg says about herself and Granny Weatherwax in Terry Pratchett's Discworld story, "The Sea and Little Fishes":
"I'm the one who's nat'rally talented. Us Oggs've got witchcraft in our blood. I never really had to sweat at it. Esme, now...she's got a bit, it's true, but it ain't a lot. She just makes it work harder'n hell."
So what kind of magic IS Peter good at?
In addition to Potions (which has the canonical Voldemort seal of approval), I suspect he's also good at Herbology. A lot of plants are used in potions, ointments and salves, which would also fit the image of Peter as someone with a knack for Healing.
And, of course, we know that Peter is excellent at Transfiguration.
A yowl arises from half the fandom. "He can't be THAT good at Transfiguration! He had to have help with the Animagus spell!"
Remus is the one who tells Harry that Peter had to have help--not Sirius. Remus would only know second-hand or third-hand whether Peter had to have help or not, because he didn't participate in the Animagus spellwork. James, Sirius and Peter did.
I can't see a fifteen-year-old Peter telling Remus something so potentially humiliating. There's no way now to confirm what James did or didn't say. And, as we've already witnessed, Sirius isn't the most objective witness on Peter's behalf.
However, even if Remus is right and Peter did need help, that does not take away from the fact that, at fifteen, Peter was one of the three youngest wizards ever to become an Animagus.
Full-grown, fully qualified, even highly gifted wizards have trouble casting the Animagus spell successfully. And not only is it difficult to the point of virtual impossibility, it's also dangerous. People get stuck between animal and human. People die.
And Peter, James and Sirius cast the spell successfully. At the age of fifteen. The three canonically youngest Animagi in all of magical history--roughly four thousand years, according to the Lexicon.
Let me repeat--the three youngest in four thousand YEARS.
Those three boys had to be extremely good wizards to accomplish something like that--Peter no less than James and Sirius.
So why do so many fans minimise Peter's intelligence and accomplishments?
Part of it, I fear, is due to Peter's appearance in the books. He is generally portrayed as sycophantic or grovelling, someone desperate to appease others stronger than himself. If he had been better at magic, the notion seems to run, he wouldn't have needed to appease anyone.
To be fair, we never do see Peter at his best in Rowling's writings. The one time that we see him as a boy, he is, like the rest of his crowd, laughing at Snape being tormented.
With all due respect, however, this doesn't make him a sycophant or a toady. It just makes him a fifteen-year-old boy hanging around with his friends. It's not uncommon for a person to behave differently in a group than he does on his own.
And the other times we see Peter (in human form, at least), he is generally in a state of terror--afraid of his former friends, afraid of the Death Eaters, afraid of Voldemort.
Peter's terror doesn't mean that he's brainless or helpless, however. After all, his abilities are part of the reason that Voldemort keeps him around (as well as being Exhibit A of how Voldemort and the Death Eaters can shatter a person). As a Death Eater, Peter has almost certainly learned how to kill--and, as a Healer (for he does treat and tend Voldemort), he would know anatomy.
Think about some of the possibilities. "Accio heart!" "Mobilicerebrum!"
Yes, Peter certainly has knowledge, training and power. What he doesn't have is confidence. Death Eater Peter has no faith in himself left--certainly none to toot his own horn. Those who knew him best are no longer inclined to remember his strengths. Voldemort, as much as he needs Peter--perhaps BECAUSE he needs Peter--clearly despises him. And most fans of the Potterverse are, sadly, disinclined to look beyond Peter's self-loathing and the contempt of his once and present allies. Consequently, Peter tends to be seen as if in a distorting mirror by those who are unaware that the distortion exists.