I agree that your kitty was probably born outside, and not socialized to humans during those first two months. Jen's idea is very good about getting her spayed (they can be done as early as 2 months, and should be done by six) and putting her into a small room where you can prevent her from hiding so much. She has evidently bonded to your other cat so well, she feels she doesn't need to interact with you. I'd schedule her for a spay (whoever said two years is very misinformed ) and either crate her in a large cage, or a small sparsely furnished room where she can't hide. Also be very careful of her around screens that are flimsy. If she feels panicked, and the window screens aren't strong, a spooked kitty can go right through them. The sooner you start working with her one on one, the better.