It all depends on whether Kathy and you are the subject or the
object. Just as you would use "I" as a subject and "me" as an
object, you would use "Kathy and I" as a subject and "Kathy and me"
as an object. If you are not sure which to use, try it with just
"I" or "me" first.
Here are some examples:
I will meet you at the station. ("I" is the subject.)
Kathy and I will meet you at the station. ("Kathy and I" is the
subject.)
This has been such a rewarding experience for me. ("Me" is the
object.)
This has been such a rewarding experience for Kathy and me
("Kathy and me" is the object.)