5 month old kittens can be pretty rotten.
I think a companion would be great for Casper. I have a 4 month old foster kitten here, and she plays constantly with my feral female cat. She uses up all her energy that way, so she is pretty good ...well other than digging in my cactus and thowing stones all over the place, and knocking things off my end tables.
A female kitten would be just the thing for Casper! Just make sure it's around the same age.