Definition:
(Noun) a behavioral attribute that is distinctive and peculiar to an individual.

In no particular order...

Lily Evans:
Reading late at night which requires her to keep awake so she consumes a lot of coffee hence addicted to caffeine.

Remus Lupin:
Craves a lot of tea and frequently smokes in between. Kinda odd but he's accustomed to smoking and drinking tea at the same time.

James Potter:
Tapping his knuckles on the desk when impatient. This tends to happen a lot which results in other people getting uncomfortable due to the continuous tap tap tap.

Sirius Black:
Chattering nonsense. He practically never shuts up and is a continuous flirt to both genders. His flirting is usually quite odd but appreciative at the same time which helps in not to entirely piss off the audience.

Peter Pettigrew:
Biting his nails. Peter gets nervous of the slightest of things and since he's an awfully clumsy person, he falls into scraps all the time. When nervous, which is very often, he would chew on his nails.

Alice Fortescue (Neville's mother):
Blowing huge bubbles from Drooble's Best Blowing Bubble Gum. She used to chew on bubble gum all the time, insisting that it was good exercise for her jaws.

Marlene McKinnon:
Rolling her eyes weirdly in either disgust or ridicule. This act of hers had become a major part of her personality and mostly people knew her as the Queen of Eye Rolling.

Narcissa Malfoy:
Scrunching up her nose as if the whole place reeks of stink and slime. This sets the impression of her narcissistic behavior on others.

Lucius Malfoy:
Kicking Dobby down the stairs whenever he feels like it. That man is an absolute cruel villain to elves. No wonder his house elves would have celebrated wildly when he was sent to Azkaban.

Luna Lovegood:
Reading magazines upside down insisting that it makes things look more interesting and the words more easier to decipher for her.

Harry Potter:
Well that's much more of a habit for him but yeah, here goes. Being the hero. Sacrificing for others and other noble heroic stuff.

Hermione Granger:
Chewing pencils. When she would get sucked up in a problem in studies she would subconsciously chew at the back of her pencil. And yes she doesn't always use quills since she finds it easier to use pencils for the rest of her work other than assignments.

Lavender Brown:
Getting girly and blush machine type at the most indifferent of things.

Parvati Patil:
Relating everything that happens in her life to what Professor Trelawney says. She believes too much in Divination and Professor Trelawney.

[These aren't all so I'll add more characters shortly.]