"Master Sirius ran away, good riddance, for he was a bad boy and broke my Mistress’s heart with his lawless ways."
-- Kreacher (DH10)
“Of course, Voldemort would have considered the ways of house-elves far beneath his notice . . . It would never have occurred to him that they might have magic that he didn’t.”
-- Hermione (DH10)
Kreacher was an aged House-Elf whose family served the Black family for generations. He was devoted to Sirius Black’s mother, who died in 1985 but whose portrait still shrieks orders from the wall of the front hall of the Black family home, number twelve, Grimmauld Place. Kreacher continued to live there after her death, although he did not do much upkeep in the years before Sirius Black offered the house to theOrder of the Phoenix for use as headquarters, an action which Kreacher resented greatly. He was very rude to the members of the Order of the Phoenix in residence, and tried to stop them from throwing away the many Dark items with which the house was filled, even going so far as to rescue some objects (OP4, OP6, OP9, OP23).
Sirius treated Kreacher disrespectfully, though Dumbledore advised others to be tolerant. Shortly before Christmas of 1995, Sirius accidentally gave Kreacher permission to leave the house, whereupon Kreacher temporarily left Grimmauld Place and went to the nearest Black family connection whom he still respected, Sirius’ cousin Narcissa Malfoy. Kreacher then proceeded to betray the Order as much as he was able, and eventually tricked Harry into thinking that Sirius was at the Department of Mysteries, which led to Sirius’ death (OP22, OP24,OP32, OP37). Harry, who was devastated by the death of his godfather, was furious that Kreacher betrayed him without remorse, but Dumbledore, in agreement with Hermione, felt that Sirius had been rather cruel and contemptuous of the house-elf, and never earned his loyalty as he should have (OP37).
To Kreacher’s horror, Sirius left all his belongings to Harry in his will, so Harry is Kreacher’s master now (HBP3). Rather than deal with him on a daily basis, Harry commanded Kreacher to work at Hogwarts, although Harry is still Kreacher’s master (HBP19). When Harry, Hermione and Ron went on the run from the Death Eaters and hid at Grimmauld Place, Harry realized that the fake Horcrux locket he and Dumbledore had removed from the sea cave belonged to Regulus Black. He asked Kreacher to tell them the tale of how Voldemort had forced the house-elf to drink the evil potion in the cave, and abandoned him there. Later, Kreacher had returned with Regulus to retrieve the real Horcrux and leave the decoy locket with a note for Voldemort signed “R.A.B” for “Regulus Arcturus Black” (DH10).
Harry finally won Kreacher over by giving him the fake locket from Regulus to keep as his own, and asking him to find Mundungus Fletcher, whom Kreacher had seen stealing the real locket as he trashed Grimmauld Place looking for things to sell. Kreacher returned with Mundungus who admitted the theft, but said Dolores Umbridge had taken the locket from him (DH11). After these events, Kreacher was much more kindly disposed towards Harry and his friends, cooking delicious food for them to eat, washing and ironing their clothes, and finally accepting Harry as his true master (DH12).
As the case of Kreacher shows, it’s very important for the wizards to know if their house-elf is really loyal to them or not, because these creatures see and hear things, and can collect information and eventually use it against their masters. With Kreacher as with Dobby, kindness and respect worked much better than anger or contempt in earning that innate house-elf loyalty.
Kreacher was last seen at the Battle of Hogwarts, leading the other house-elves in a charge and shouting: “Fight! Fight! Fight for my Master, defender of the house-elves! Fight the Dark Lord, in the name of brave Regulus! Fight!” (DH36)
Sneaky, able to survive attacks by Inferi, and able to disapparate from dangerous places
'Kreacher' is a homonym for the word 'creature;' the name suggests both the elf's animal nature and his age (in its similarity to the word "creaky").
Kreacher was included in the films for Order of the Phoenix and Deathly Hallows Part 1. At first, Kreacher was cut from OP, but producer David Heyman explained that J.K. Rowling said they should include him because of his role in finding the real Slytherin Horcrux in DH:
"We took Kreacher out, and Jo said, 'Listen guys, you don't have to put Kreacher back, but I'm just telling you, if you want to kind of keep a thread going for six and seven, you might want Kreacher to come back.' She basically told us Kreacher plays a role in Seven, in a sense. She hinted. We thought about it for five seconds, and he came back." (MTV Movie Blog "Kreacher Comfort" 6/25/07).
From the Web
Sirius Black versus Kreacher: Who’s to blame? by Ebrosnan