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.
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 him from falling off the airplane.
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.