* Kasia (pronounced "Kash-ah") - This is a common Polish nickname for Katarzyna (Katherine), which is a widely known name in Poland. It's the most natural-sounding equivalent for "Kathy" in Polish.
* Kacper (pronounced "Kats-per") - While this is a masculine name, it is the closest phonetic equivalent for "Kathy" in Polish.
It's important to consider the context and your relationship with the person when choosing the most appropriate way to spell Kathy in Polish.
If you're unsure, it's always best to ask the person directly how they prefer to be addressed.