Kreacher

Kreacher is an aged house-elf who serves Harry Potter. For most of his life, Kreacher served various members of the House of Black, including Sirius Black and his mother (whom Kreacher remembers fondly and still calls "his mistress"), who died in 1985, but whose portrait still shrieks orders from the wall in the front hall of the Black house.

Under the Order
Kreacher was nasty and rude to the inhabitants of 12 Grimmauld Place and didn't do much actual upkeep. He had been known to steal Black family heirlooms and hide them in his den in the kitchen cupboard.

Kreacher was loyal only to pure-blood wizards in general, Mrs. Black in particular. He was openly hostile to the entire Order of the Phoenix, but was forced to serve Sirius Black, whom he regarded as a blood traitor. Kreacher was considered somewhat of a liability by the Order of the Phoenix, who used the Blacks' house as headquarters; as a house-elf, Kreacher overheard things of a sensitive nature. He was forbidden, however, to speak of the most important things, but sometime in December 1995, Sirius accidentally told him to leave. Taking that to literally mean leave the house, he went to the Malfoys, close relatives of the Blacks, and told Narcissa Malfoy (née Black) several details that were not considered to be of any importance, yet led to Sirius' demise.

Under Harry Potter
Upon his death, Sirius willed all his belongings (including Kreacher) to Harry. Harry, who had no desire to have anything to do with Kreacher, and to keep him out of the Order's way, ordered him to work at Hogwarts with the other house-elves, where he could keep a close watch on him. This eventually turned out to Harry's advantage, as Kreacher &mdash; along with Dobby, another house-elf working at Hogwarts &mdash; tailed Draco Malfoy for him when he suspected Malfoy to be up to no good. For Christmas in 1996, Kreacher gave Harry a box of maggots.

In 1997, Harry made Kreacher tell him about Regulus Arcturus Black, who was the RAB who took the locket that was Voldemort's horcrux. Kreacher was ordered by Regulus to destroy it, but was unable to open it. He told Harry that Mundungus Fletcher stole the real locket, and was rewarded by Harry with Regulus's false locket. Kreacher's joy at being given a Black family "heirloom" as his own drastically changed his opinion of Harry, whom he starts to serve properly. Hermione Granger turns out to be right about Kreacher; he was nice to those who treat him well.

"Fight, Fight for my master, the defender of the House elves. Fight for brave Regulus!"

- Kreacher leading the house-elves during the Second Battle of Hogwarts

He was leading the house-elves of Hogwarts in battle against the Death Eaters during the attack on Hogwarts, for the sake of the memory of his long-dead master Regulus, because he reminded that, although he was a Death Eater, he ended fighting Lord Voldemort. It is not known whether he survived the battle but Harry who wants to go to his bed after the Battle of Hogwarts wonders if Kreacher could bring him a sandwich, implying that he did indeed survive the battle.

Behind the Scenes
Kreacher's name is most likely derived from the word "creature."

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows