I hate to be the bearer of bad news, but if cutting her is even a possibility, it probably means her presence isn’t required, in which case you should probably just cut her. The fact that you are having trouble developing her and keep forgetting her is a good sign that she doesn’t belong there. What I would do is write down what is important about her and file it away for a future story. You’d be amazed how often a character who isn’t working out in one story ends up being a perfect fit for another story. I know it isn’t easy to cut a character, especially when you’re attached to them and can envision them in your story, but sometimes it must be done. I think this is one of the places the adage “kill your darlings” comes from.
If you’re not sure whether she serves a purpose, take a good long look at your story and ask yourself honestly whether the plot would be affected if you cut her. If the answer is ‘no,’ cut her, but if it’s 'yes,’ here are some things you can do to help you develop her.
1) Fill out a character development worksheet. These are a great way to discover new things about your character, and they’ll also help you establish facts that can be pulled from later to add depth to your character. Try this one, this one, and this one.
2) Cannibalize another character. When you’re dealing with a lot of characters in one story, and one is falling flat, take a look at your other characters and see first whether all of them are necessary, and second, whether any of them have traits that might be better suited to your character that is lacking.
3) Time Travel Camera Crew! This one is fun–choose a point in time just prior to when your story starts, and pretend you are a camera crew following your character through a normal day. Write down everything that happens and everything the camera sees, from what she does to how she acts. You can even do little reaction interviews where she divulges her feelings about things that happened. Think about it like an episode of The Office or a reality show, even. This allows your character to develop organically within the universe of your story, but without the constraints of the plot.
4) Establish inner conflict and flaws. Even secondary characters need to have inner conflict and flaws. An inner conflict will give your secondary characters their own motivation as they participate in and affect the plot. Flaws will not only make them believable, but can also be used to create tension and obstacles.
5) Growth makes a character dynamic. It’s not enough to make a character well-rounded trait-wise. Characters, even secondary, should be dynamic through their own growth over time. In other words, give them a character arc. Just make sure it doesn’t take away from the main character’s journey. The complexity of the arc will depend on how important the character is. Even minor characters can undergo an emotional change by the end of the story–you just wouldn’t want to spend a lot of time on it. For more important secondary characters, though, you can find ways to tie their growth into the story–again, so long as it doesn’t overwhelm the protagonist’s character arc.
Resources:
Five Steps to Building a Believable Character Arc
How to Create a Strong Character: Give Them Flaws
Types of Characters in Fiction
