When writing a story, it is useful to have an outline of your character on paper. Here's an example of one that I've used before.
Eye color: Hair color:
Quirks: Bad Habits:
Things embarrassed by:
The list can actually go on and on, but these are the ones I usually answer right away and use to start my writing process.
For a more comprehensive list, pinterest has some good examples or check out books like Marc McCutcheon's Building Believable Characters which includes many useful ways to develop characters.