Kreacher was the house elf to the Black family. He like his owner (Sirius' mother) only believed in pureblood wizarding families. When Mrs. Black died, Sirius became his owner. He had a dislike of Harry from the moment that he walked into 12 Grimmauld Place, but after Sirius was killed Harry became his owner. Although he could not stand Harry one bit he had to do what he was told. In the final book (The Deathly Hallows) Kreacher comes to appreciate and respect Harry after he gives him his old master's locket; even so much that he fights for Harry in the end.