This article is about the relationship between Kaeloo and Mr. Cat's relationship in general. If you were looking for an article about the romantic aspects of their relationship, go here.
Kaeloo and Mr. Cat have a very complicated relationship. While they do fight a lot, they care a lot about each other. Mr. Cat actually has a crush on Kaeloo, which is heavily implied to be reciprocated.
Kaeloo sees Mr. Cat as a good friend despite his cruel and sadistic nature. She tries to be very kind to him, and is somewhat aware of the hidden softer side of him. She tries not to transform because she doesn't want to hurt him, though once she does transform she is usually brutal when dealing with him. Sometimes, Kaeloo isn't very nice to Mr. Cat, like in "Let's Play Courtroom Drama" when she emotionally manipulates him in order to get a confession from him and then beats him up, or in "Let's Play Ki Cé Ka Raison" where she sets him up on a game show which is rigged so he will lose, and even if he wins he suffers. Kaeloo may reciprocate Mr. Cat's feelings for her, though she is extremely oblivious to his crush (which everyone else in Smileyland seems to have figured out). Almost any time he confesses his love to her, she assumes that he is joking or playing a game.
As Bad Kaeloo, she appears to enjoy torturing him, though she may still like him even in her transformed state. In "Let's Play Spies", she hugged him, Stumpy and Quack Quack. In "Let's Play Peace, Man!", she actually forcibly kissed him offscreen (and after detransforming asked him if they could start all over again).
Mr. Cat is sometimes very mean to Kaeloo, but he seems to care for her more than he does for his other friends. He repeatedly angers her by tormenting Quack Quack and doing various other things she doesn't like, but this is in fact because he enjoys seeing her transform due to his crush on her. He does have a crush on her, but he was repulsed at the idea of being kissed by her in "Let's Play Peace, Man!". In other episodes, he asks her to kiss him, which she refuses to do for various reasons. He dislikes seeing her cry, attempts to comfort her if she does, and expresses remorse if he was responsible for it in any way. He gets very angry if anyone tries to hurt her, and will immediately defend her, like in "Let's Play Household Hazards" when he tries to shoot Olaf with a bazooka just for blowing a hairdryer in Kaeloo's eyes or "Let's Play Top Models" when he helps her get Pretty humiliated on the news because she insulted Kaeloo. If she is in any form of danger, he will try to protect her, as seen in "Let's Play the Quest for the Wholly Gruel" when he tries to save her from being crushed to death by a giant meteorite (fortunately, the meteorite in question never landed).