I think where a lot of writers (sadly, of both genders) go wrong is where they've decided to put a character into a book just to seem like they're not being sexist/racist/latestfauxliberalbuzzwordist. This should go without saying: if you're forcing a character into a novel or short story that doesn't have any internally consistent reason to be in that prose, then you've made a mistake. For the love of God, don't force any character into the prose for a reason that was generated externally to the prose (e.g. 'my friend liked an idea', or 'I don't want to put off x'); place characters into the prose only if they have a good reason for being there, and you're able to understand their character as well as all the other characters.
Yes, yes, yes! You took the words right out of my mouth. If there is no reason for a woman to be in the story, then don't bother. Same as if there is no reason for a man to be in a story, don't bother. What does she contribute by being a woman in that group? What talents/traits/abilities does she need to bring? Is she a softening influence? An object of envy to fight over? A matriarch? The mother of all 'dem boys? A little sister getting in the way? A big sister managing the bunch? The local witch queen or the good witch bailing them all out? Is she the peacemaker or the manipulative instigator? There are many, many roles for women to play that are essential in society without having to carry either a sword or a spoon.
1] What's her background like? What's her society like? How much power are women typically permitted in her society? How much power did her parents permit her?
Excellent foundational questions. Her society dictates her influence, but not her abilities to influence. Scarlett O'Hara, one of the great female characters of all time, was locked out of power positions in her society by dint of her gender, but it did not stop her manipulating every male around her to get her way--in ways perfectly recognized and accepted by the men. I recall that it was burqa-clad women in Egypt who saved the female reporter who was being gang-raped this spring. They shamed the men into letting her go, in a society where women are severely repressed. Women do have a sort of power in every society (ever read Lysistrata?
) Just because she is physically weaker doesn't mean she's helpless, so think about what role you NEED her to play in your story, and give her the wherewithal to do it--either brains, looks, physical abilities, magic, determination, or whatever. Then make her true to herself in her thoughts, words, and deeds. Is she allowed to go off and have a crying jag in private even if she's leading the group? Sure. But it has to be in her nature to do so. Just because she stiff-lipped in public doesn't mean she's not bleeding inside.
1]Here's the absolute most important bit of the plan: understanding how your fictitious society will react to the character you've just made; it can be the most original, interesting, and downright refreshing character anyone's ever written, but if it's being reacted to by other characters in a way one wouldn't expect, then it's worthless
This is true of every character. They have only as much power as society lets them have, which includes the Evil Overlord who has spent his life conquering everything around him. If he is still in power, it is because sufficient numbers of people have not grown courageous enough or angry enough to go after him, or made the effort to find weapons to use that will bring him down. How your society reacts to your characters must be consistent. If she is unconventional, then her peers will be shocked, perhaps unforgivingly so, or perhaps secretly envious of her daring. The men may be patronizing, or sourly incredulous that she can actually deliver on what she says. Some may worship her. Some may try to assassinate her for being different and "uppity". Some may despise her weaknesses and refuse to recognize her good points. In all cases, the reactions should be shaped by individual personality as well as by societal norms/variations.
Nice three-point list, by the way.