>>312903671
Why the hell is everyone doing Koikatsu for their first model now? It's not a great place to start, and most people with experience avoid working with them because they're not good bases at all outside of their porn game.
You're pasting in the hair from another .blend file? I'd suggest hitting "fix model" with "Model" selected as your armature in Cats so that the bones will be named the same, then selecting the hair model and any other of your meshes, and joining them together. You can separate them again later, but by joining the hair to one of your original meshes, it ensures that there won't be any scaling issues and that it's parented to the original armature.