What should I do about my foster cat?

Keeping her confined in the bathroom is not a good idea. Obviously she does not like the arrangement. I understand your predicament as I have an older dominant male who doesn't like many intruders in his environment. You didn't say if Kelsey is older. If she is that might be part of the problem. My 8 year old will accept kittens but not older cats, male or female. Still, we have managed to foster 7 cats/kittens this year and have found them good homes. I would try and place her in a home without the issues you have. Kelsey is being traumatized by being confined to the bathroom or she wouldn't cry like she does. Feel free to e-mail me if you think it will help. Good luck and blessings to all of you.