Kreacher is an antagonist in the three Harry Potter books, Harry Potter and the Order of the Phoenix, Harry Potter and the Half-Blood Prince, and the first part of Harry Potter and the Deathly Hallows. A vile, repulsive, ugly house-elf, Kreacher was utterly loyal to the Black family, bordering on mental illness, and he was sickeningly devoted to his deceased ancestral masters, including Sirius' mother. Kreacher was arrogant, cruel, cold and unfriendly. He made slanderous insults to every visitor of the Black House, such as Hermione and Ron. Kreacher was sympathized by Hermione who saw house-elves as innocent slaves, but she was wrong because Kreacher, unlike many house-elves, he was highly intelligent, but very old, so he had no brute strength, but he had mental manipulation and knew of treachery. Kreacher longed to be with Narcissa Malfoy and her family because they treated him better than Sirius. Kreacher hated Sirius, because Sirius hated the last living memory of his awful family life. Kreacher insulted Sirius many times after his murder.
Kreacher was notably one of the main antagonists in the fifth book, Harry Potter and the Order of the Phoenix. He was upset at Sirius' ordering and sweeping of the House because in his view it was all his heirlooms they were demolishing, and Kreacher had let the House go to decay in ten years and parasites had been breeding. The Black family had a cruel tradition of decapitating old and weak house-elves and sticking their heads up on a plaque; yet Kreacher was so mentally ill he considered this an honor and he wanted to be nailed up with his ancestors.
Kreacher made his biggest villainous act when he betrayed Sirius when the master kicked him out of the kitchen, meaning him to leave the kitchen, but Kreacher took the meaning as "leave this House." Kreacher eavesdropped on the account Harry gave of Arthur Weasley's attack, and Kreacher later told Narcissa and Bellatrix Lestrange what the Order's plans were and how many were in the Order. Kreacher was not however fully able to betray the Order because he wasn't their protector and couldn't say where the House of Black was. Kreacher however sold enough information to get Narcissa knowing that Harry loved Sirius and would go to protect him from Voldemort in a possible hostage crisis. So, Narcissa told Voldemort of the information and he planted a false image in Harry's head, thanks to Kreacher's treachery.
Harry would come to hate Kreacher utterly for this, he would not look at him. He wanted to be rid of Kreacher. He wanted to have Kreacher go to Hogwarts, knowing he couldn't be freed, and work in the kitchens.
Later, Kreacher reluctantly agreed to be Harry's servant, despite the fact that he said he would rather serve Draco Malfoy, and called Harry a brat. Kreacher also was rude to Hermione even when she complimented him for his efforts in tracking Malfoy, Kreacher just said "the Mudblood is speaking to Kreacher, Kreacher cannot hear." Oh yes, Kreacher always spoke in third person, referring to himself, further hinting of his mental illness.
Kreacher however, despite betraying Sirius, Harry and the entire Order, somewhat redeemed himself because he told Harry he had sided with Regulus Black, Sirius' brother, to get a Horcrux and destroy it. Kreacher had been abused by Lord Voldemort and he had watched Regulus be taken by Inferi and drowned. Kreacher made it back with the Horcrux. But nothing destroyed it. It would later be stolen by Dolores Umbridge. Kreacher was valuable to Harry in the quest and tracked Fletcher to ask him who had stolen the locket.
Kreacher later forgave Hermione and Harry and Ron and Harry forgave Kreacher, and Kreacher fought in the Battle of Hogwarts, leading the house-elves into battle. He survived.