Many are little more than a plot device than a person/character. A quick tip, if she can be replaced with a magical object the hero wants, there’s a problem.
I’m not saying all women have to be butt kicking heroes even though I tend to like that type of character. It’s perfectly fine to have a female character who doesn’t fight and isn’t one of the guys, but it doesn’t mean she has to be an object to be jerked around by the hero or villain. She needs to have her own goals, her own thoughts, and her own choices, and a spark of courage. (Note: Her goal should be something other than getting the guy.) Even some girls who can legitimately fight back still end up being cardboard cutout DiDs because the only reason they’re needed is so the hero has someone to rescue.
Princess Leia is a good example of how a girl who gets rescued can avoid being a DiD. Unlike most of DiDs, she has her own goal that doesn’t involve romance. She wants to beat the Empire and end their tyranny. The second Luke lets her out of prison, she tries to take charge of the situation, something Han really didn’t like. If the heroes had been killed, Leia would have kept fighting. Her life didn’t revolve around someone else.
A way to check to see if the character is needed is trying various exercises. One trick is to imagine the woman as a man. If a man were in her situation, what would he do? Would he be necessary to the plot? (Imagine Leia as Lee. The movie could have progressed in almost the same manner.) This might not work for everyone, but it will help you recognize if you’ve got a character that may annoy girls who want to read about strong female characters. It might also let you know if you’ve got a situation where the character would be unneeded if she wasn’t a romantic interest. Another exercise is to imagine what would happen if the main male character dies. Would she be able to take his place, or would she just crawl off in a hole to die?
I’m not saying all characters need to be tough like Leia or Black Widow, but they should have their own goals and thoughts. This will help too keep them from turning into a plot device. If they’re the main character, this is probably the most important thing you can do to keep them from being bland. A main character of either gender who never makes choices or does things by her/himself is pretty boring.
Another thing to remember is that the girl should have some measure of courage. Yes, fear is natural, but in the end, she should overcome it, at least enough to do what’s needed. People like reading about those who rise above fear, not those who are still controlled by it when the book ends.
In the end, remember every character is a person. They have their own objective. They’re not objects with the purpose of serving the hero’s story.