I feel for you! A few random thoughts, potentially not articulated as well as I would like given more time!
- can you approach your manager with your concerns - either as a 'I deserve a raise because I am bearing more responsibility' or as a 'I'm not sure this person can do the job they were hired to do given these points'?
- did you manager hire her? Did somebody who understands design / this position's role sit in on her interview? If not, perhaps she's pulled a swifty?
I once interviewed a girl who cold-called for a job, and she had a nice portfolio, very nice work. As we were a little swamped at the time, I suggested she come in for a day's paid freelancing to see if she liked us, we liked her, and with a view to taking some of our overflow in the short term.
I sat with her and talked through her portfolio... but when it came to sitting at the computer and creating work it became screamingly obvious that she must have lied, and the portfolio work was not hers. There was such a disconnect between what was presented and what she was able to do (both concepting ideas, and actually using the programs to execute those ideas).
She was such a nice girl, but she'd been looking for a job for two or so years and I think desperation got to her. She must have figured that if she at least got her foot in the door via 'her' portfolio, she'd be able to fudge her way through the position, getting help and learning.
The problem is that she doesn't have any layout skills, or, based upon what I've seen of her portfolio, she doesn't apply her knowledge here at work.
I wonder if your person is a bit the same, from what you've said?