Mr. Cat usually tortures Quack Quack because he is indestructible and therefore able to survive any injury, but they still see each other as friends. In fact, their friendship is so strong that they even have their own special secret handshake.
Mr. Cat does various horrible things to Quack Quack, ranging from shooting him with a bazooka, tricking him into breaking up with his girlfriend on Valentine's Day or outright sending him to Hell. Nevertheless, he still cares about him. For example, in "Let's Play Bye Bye, Yoghurt", he gives Quack Quack a new yogurt to replace the one that expired, and in "Let's Play Castaways" he saves the duck from falling off of an airplane. In fact, in "Let's Play Gaga of the Throne", the prospect of Quack Quack dying once and for all without being revived was enough to make him start crying.
Quack Quack usually forgives Mr. Cat for the latter's constant abuse of him, but in episodes like "Let's Play Figurines", "Let's Play Ki Cé Ka Raison" and "Let's Play Clone Party" he doesn't hesitate to get revenge on him. This rarely ever happens, though, and he seems to actually care about Mr. Cat.