Last week, I got two kittens (SJ and JE) from a local shelter, a bonded male and female pair.
They were both very fearful at first, but I attempted to manage this by making sure they had their own room with all of their basic needs cared for: food, litter box, cat tree, heated bed -- and a large human bed to hide under. At first, they hid under the bed and one of them hissed at me when I approached, so I gave them space. They have been using the litter box, generally with no problem.
Eventually, through giving them treats and feeding them good food (I've been giving them high-quality wet food and boiling chicken for them), I've gotten them to come out of their shell a little bit. The male kitten has been much more adventurous and vocal and eats without issue. I've even gotten him to let me pet him a few times (though I haven't really tried picking him up) and to step out of the main room I have reserved for them for a little bit before he scurried back in. I'm not too worried about him.
The female kitten is very quiet (she almost never says anything), generally less-willing to try new things out, more likely to hide under the bed and less likely to approach me. She's also a little clumsier than her brother -- she's taken a few attempts to jump onto a high ledge, and she has a little trouble finding treats on the ground and keeping them in her mouth. She's also got a much more sensitive stomach and has had diarrhea after I gave them both new foods (which I have since cut back on). I still haven't been able to pet her, but I have been able to coax her out of her comfort zone a bit with treats and with playtime; she loves chasing little felt mice around the room.
Today, I went in for playtime with the kittens, and she discovered something that I hadn't -- the wand toy that I had been using had a plastic piece in it that she inadvertently pulled out, which made the toy start squeaking. This freaked both of them out, and she ran under the bed to hide, and I couldn't coax her back out with toys or treats, although five minutes later, her brother was fine and chasing other toys and eating dinner and behaving normally towards me. Meanwhile, the female kitten has gone deep under the bed and only stares unblinkingly back at me when I look at her or try to coax her out.
I have read other posts (like this one and this one) and I don't know much about these kittens' past aside from the fact that they are shelter animals that were found in a basement, but I'm a little worried about her, and I'm dreading taking them to the vet next week for an introductory visit. I'm happy to be patient and comforting to them both, and I accept that I may just have an especially skittish lady cat, but I also wanted to ask how much of this was normal and what I should prepare for in the future.