Being a strong person, either shinigami or Quincy (who you yourself want to be), you are the only one who is respected by Zangetsu and he does not even mind that you will be stronger than him, which means that you are a powerful king/queen in his eyes.

But still, he softens up a lot with you, sometimes not even looking like himself and just snuggling up to you, like a kitten snuggling up to its owner. It's funny, but he doesn't mind until someone else tells him about it.

One day Ichigo himself just said that Zangetsu was too soft with you, and Zangetsu threw a tantrum, saying that he was strong, cruel and bloodthirsty, and not a little white kitten. This tantrum lasted for about two hours, and at the end of his tirade, he told you in a rough and low growl: "You're killing the tough guy personality in me," but you just shrugged and said warmly again:"Who is my good kitten?" He relents again and falls into your arms, happily purring "Meeeee!"