Actually, I prefer to develop their characters throughout the story than to write it in one go.

Interesting characters always have flaws. You could put a personal twist on a common character too :)

Or you could take characters you like already and take one-two personality traits from them and then fill in the blanks for the rest of their personality. Say I use Captain Kirk, I might take his confidence and no-lose scenario personality traits and then fill in the blanks by adding intelligent, rash, temperamental, or any other personality traits where I could add flaws or flourishes.

But the big thing about characterization that every person should know is that everyone must have a reason for why they do things.

I usually take a few traits depending on what character I'm making and then expand on them. Say if I take intelligent, aloof, and experienced, I would build on those characteristics by giving examples on why and what the character does to have those traits. If the character's intelligent, I could mention how she's intelligent and what type of intelligence she has like being book smart or street smart or even a bit of both. Then I would go onto the next trait and so on. And then when I'm done going over the each trait, I go back to make sure the character has flaws, probably highlighting at least one flaw per character trait.

So if the person is aloof, she might be good at dealing with her problems on her own or is just uninterested in things to cause her to freak out so she has good compartmentalization skills, but she might be so aloof that she has trouble making friends or connecting to people because she doesn't let many see how she really feels about things or looks cold to others.

Look into the psychology of things. Ask yourself "Why does my character do this" or "What would their reaction be to this if they were this type of person?"

And then make sure your character doesn't have conflicting personality traits like being a naive airhead and then saying they have wit and sarcasm as well. Traits like that don't go together because if the character's an airhead, than they probably aren't smart enough to come up with witty comebacks and be sarcastic about things.

Don't just focus on the traits she has, also think about how she responds to different situations, what her personal values and beliefs are, etc. For example, if she's smart, that only says so much about her as a person. Not all smart people are the same. Does she like to show off her intelligence? Does she keep her thoughts to herself? How do other people perceive her? What are her own thoughts about her intelligence? Is she confident in her intelligence? Does she use her intelligence for selfish goals, to help others, or a combination of both? And so on.

have flaws.
have bad and good habits
a perfect girl and a bad girl is too common.
and have some unique habits like your own habits