The question has been raised on our boards whether or not Cat could be declared as a defender along with some other character against an attacking character with the keyword "Joust".
My guess is she cannot, even though she is immune, because the other character - which is not immune - cannot be declared at the same time as Cat'. When you look at the restriction to declare him as a defender, you count the total number of character you want to declare and Cat' is in that count - making it 2, not 1, even though she is immune - so the 2nd character is not legal to be declared as a defender, so it is also not legal to declare both.
However I'd say you may declare Cat 'o the canals + Jaqen H'gar in case he got Immunity to opponent's card effects through your opponent's dead Cat'...