A forum post I made about some quick ways to give life to a new character. Edited slightly to be more generalized.

-------------

1) It's *really* hard to develop characters in a total vacuum. Characters are initially best served being compared and interacted against other existing OCs or canon characters you know of. I like to imagine how my boys/girls/neithers will fit into my existing settings' "OC ecosystems". Simply form relationships with your other characters- can be hate, love, both, indifference, etc and how do they bounce off of each other?

2) A good approach I like for making characters fleshed out fast is mentally running her through scenarios of awful things happening to her and thinking about how she would react. Or pick a good story you'd think she'd belong in and see how she'd handle things differently from the characters in there- kind of a fanfic-seeming exercise, but it has its purposes.

3) Come up with a fictional timeline/history of her education and training (this will require research!), as well as any enduring or particularly vivid relationships she's had with someone else, also and her thought process during and after both events.

4) Sometimes I like to assign to one primary principle to a character, which they live their life by and three secondary values they strive for as long as it doesn't conflict with the former. For example, The Grand Duck primarily values Devotion- to universal annihilation. His secondary values are Bravery, Pride (he is a very judgmental sort) and Power. Everything he does revolves around this and all his friends had better display these if they want to stay friends. Keep the principle secret or implied though, it seems kind of cheesy to state it outright on the profile :P Then at that point, come up with a list of ways your character displays these traits and what happened to them to create these traits. This is a cheap way to jumpstart "empty" characters, in my opinion, but is probably not as good for advanced fleshing-out of an already stable character.

Psyduck- Devotion to annihilation- Demonstrated by him doing pretty much nothing else but break things, exhorting people to break things, as well as him eschewing all creature comforts and material possessions unless they can help with his cause. Caused by a deep and moving philosophical realization one day that life is effectively random and self-defeating.

Bravery- Caused by his experiences chasing down cowardly survivors of his attacks on villages and his general insanity. Demonstrated by his general disregard for self-safety and extreme aggressiveness in the heat of battle and also  his insistence that his allies be as aggressive as he is. Is overcome by his primary principle, Devotion, in that he is ready to die, but only if it advances his ultimate goal. He sees no value in a senseless death outside of battle.

Pride- Caused by his belief in the superiority of his "annihilist" philosophy and that most people don't share his enlightenment, so they must be uneducated about the truth! Demonstrated by his utter disregard for the value of others' lives, unless they join his cause, at which point they are incredible friends and he will do anything to help them out as long as they stay Devoted (again, primary > secondary).

Power- Caused by his need to kill people efficiently and quickly. Demonstrated by the horde of eccentric but dangerous miscreants he courts and leads. That said, he has trouble respecting "worthy adversaries" if they do not at least show token respect to his personal philosophy (again, primary > secondary)

(you'll want to be way more verbose than I am being and have multiple causes/effects of each principle)

And then you now know how your character react to basic situations based on these guiding principles.