Another kitten may help, but I think you'd want to make sure she's about the same size as Casper because I have a hunch that Casper will want to be Alpha. And if she's much smaller than him, he may hurt her.
PS For training my cats, I like to use water bottles. They get mad at the water, not me. (Except for the one who stands there and opens her mouth....) Don't know if you have tried that or not.
I'll keep thinking!