Hagrid:
the more I think about it, the more problematic the first introduction of him is. Primarily turning Dudley into a pig. That is what Death Eaters do. It is funny in the context, because the Dursleys are at that point set up as the baddies who deserver to be put down, but in the wider context of the later books it is deeply problematic. Especially as he punishes Dudley, not Vernon for Vernon's provocation.

That said, his division of the world is clearly people he respects vs people he doesn't, and that dictates his actions. But I still find that first meeting a little off key, considering.

Draco:
The comparison to Dudley sets him up as distasteful. But it is strange to see how everything he says in the first meeting is so clearly just a repetition of what he has heard from others (his father, mainly, I suppose). He is saying "the right thing" based on the circles he is used to, and does not seem aware that it is not on outside of that circle. He is trying to bond with Harry and is rejected.

And then he is rejected again in the train, while his lackeys are standing by. Like what she does with Dumbledore, Rowling gives the character first, then the background information: Ron only tells us Draco's family is Bad after Draco has shown himself to be bad, or at least contemptible, on his own.

The Weasleys:
That really is Rowling at her best. She gives such a wonderfully vivid snapshot of that family. They are nice, poor, happy, with a bustling, fussy mother, a shy sister, twins who play practical jokes and the stuck up Percy (how did he get into Gryffindor? Talk about not living up to his potential). And then Ron's description of his brothers serves not only to gives us the Weasley brothers in a nutshell, but also defines him as a character for … well, most of the books, really.

Dumbledore:
I like what Rowling did with the chocolate frog cards. We have already met Dumbledore, of course, but all we have are his character (eccentric, fan of weird candy, wise old man) -- here are his credentials. And I like how Grindewald is introduced this early, considering his importance to the last book. It reinforces the eccentricities in mentioning his enjoyment of chamber music (fairly normal) and tenpin baling (not so much?). The chocolate frog card is really quite well packed. The first time I read the book I was impressed that she had introduced Nicholas Flamel that early (my expectations were low), but now I am even more impressed.

And then there is Dumbledore's speech in the first banquet. It is glorious. "Before we begin the banquet, I would like to say a few words. And here they are: Nitwit! Blubber! Oddment! Tweak!"

Hermione:
I would say I love her if that wouldn't be tantamount to saying I love me as a child. She has all the bad habits that I have (had? maybe?) when insecure: make sure everyone knows you have read everything. She is so insecure in the beginning, and like Draco she reacts by pretending to have complete control. I haven't noticed that similarity between the two, before. Of course their ways of pretending to have control are what separates them.

She gets a little insufferable there for a while, but it is all good. "killed -- or worse, expelled". But the mountain troll episode redeems her (yes, I am stating the obvious today).

I love Ollivander.

Neville:
Poor Neville. He is just plain pathetic in the beginning. Of course, that is what makes him so brilliantly wonderful later on. Why does he love that toad? Does it have a significance that I have forgotten? Or does he just fear his grandmother?

Scabbers:
It is uncharacteristic of Pettigrew to show courage. I am wondering why he bit Goyle. Do we ever get to know? I suppose it might be some resentment because he has to hide as a rat while other ex-Death Eaters' children get to bully people and eat chocolate.

Snape:
So sinister. I like how Rowling makes use of our assumption that correlation implies causation in order to make Snape scary from the beginning (the pain in the scar just as he looks at Harry). And then the potions class…

The first time I read it, I swallowed the red herring whole (yes, I mixed the expressions on purpose). I was so sure he was the bad guy… But once that was disproved, I never believed Snape was evil again.

McGonnagall:
Her character is really only drawn when she reacts to Harry's flying without permission. Before that she is very two-dimensional: strict teacher who expects the rules to be followed. Suddenly she is revealed as someone who is able to prioritise. This is (and her lack of being Evil) is what sets her apart from Umbridge later on.

Wood:
Ok, he is two-dimensional, but I don't care. His enthusiasm for Quidditch is the only reason I care about the game.

Filch:
The more I think about it, the more I must say I am impressed with him. He has no magic ability, yet he works in that school which is full of magical students who hate him with a passion. Never mind the scary things that show up later.

Hogwarts:
I feel the school itself should be mentioned if we are talking about characterisation. It is clearly a living thing with quirks of its own. Especially in the beginning of chapter eight, but it keeps propping up throughout (ending with the Room of Requirement later on).