Kreacher Quotes in Harry Potter and the Deathly Hallows
"Elf magic isn't like wizard's magic, is it?" said Ron. "I mean, they can Apparate and Disapparate in and out of Hogwarts when we can't."
There was silence as Harry digested this. How could Voldemort have made such a mistake? But even as he thought this, Hermione spoke, and her voice was icy.
"Of course, Voldemort would have considered the ways of house-elves far beneath his notice, just like all the purebloods who treat them like animals...It would never have occurred to him that they might have magic that he didn't."
"Sirius was horrible to Kreacher, Harry, and it's no good looking like that, you know it's true. Kreacher had been alone for a long time when Sirius came to live here, and he was probably starving for a bit of affection. I'm sure 'Miss Cissy' and 'Miss Bella' were perfectly lovely to Kreacher when he turned up, so he did them a favor and told them everything they wanted to know. I've said all along that wizards would pay for how they treat house-elves. Well, Voldemort did...and so did Sirius."