My quirk is around even numbers. I hate odd numbers. When I'm reading, I'll never leave a book on chapter 8 or 10 or 14 because that means I'll have read an odd number of chapters. I always read on until 11 or 9 or whatever. If I can't because I'm running late or need to go to sleep, I'll read a few pages into the chapter so I won't be leaving it on an odd number.
These tiny OCD things are what make us human and individual. When writing realistic characters, it's important to find some quirk to make them real.
What is your personal quirk? What's your favorite character's?