"Loyal magical creatures bound to their owners as servants for life."

"Kreacher is what he has been made by wizards, Harry. Yes, he is to be pitied. His existence has been as miserable as your friend Dobby's. He was forced to do Sirius's bidding, because Sirius was the last of the family to which he was enslaved, but he felt no true loyalty to him. And whatever Kreacher's faults, it must be admitted that Sirius did nothing to make Kreacher's lot easier."

Albus Dumbledore: "Give him an order. If he has passed into your ownership, he will have to obey. If not, then we shall have to think of some other means of keeping him from his rightful mistress."
Kreacher: "Won't, won't, won't, WON'T!"
Harry Potter: "Kreacher, SHUT UP!"