He was voiced by late Timothy Bateson in Harry Potter and the Order of the Phoenix and by Simon McBurney in Harry Potter and the Deathly Hallows - Part 1, who also portrayed Chief Atlee in Mission: Impossible - Rogue Nation.
An old rude 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, Ron, Lupin, and Fletcher (not being wrong with the latter). Kreacher was sympathized by Hermione who saw house-elves as brainwashed slaves. Kreacher was absolutely brainwashed, but unlike many house-elves and more like Dobby, he was less blindly loyal and highly intelligent, but very old, so he had no brute strength. However, 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. Both Albus Dumbledore and Hermione explained to Harry the hard truth between Sirius and Kreacher: if the old elf betrayed Sirius is only because Sirius treated him badly, causing Kreacher to hate him even more and wanting to get rid of him. Apparently, what house elf like is being shown kindness or at least praise for their hard work, which Kreacher never got from Sirius.
Kreacher's favorite master was Regulus Black, who was the most kind and pleasant to him. Regulus eventually joined with the Death Eaters under Lord Voldemort, who needed a house-elf, and Regulus volunteered Kreacher, saying it would be an honour to serve the Dark Lord. Voldemort took Kreacher to the Crystal Cave and forced him to drink the potion in the basin where Salazar Slytherin's Locket, made an Horcrux, would be put in. The potion was the same one Dumbledore would drink years later and tortured Kreacher, causing him to have nightmarish delusions, intense stomach pains and made him dehydrated. Voldemort then left, leaving Kreacher to drink from the lake (as any water conjured evaporates) and die at the hands of the Inferi, so that no one would ever know about the cave. The arrogant and supremacist Voldemort, had never bothered nor cared to learn that House Elves can apparate out of locations that Wizards and Witches cannot. In fact Kreacher used his own magic, which is always underestimated by the humans and was not contained by Voldemort's enchantments on the cave. Kreacher was able to survive and Disapparate from the cave and follow his master's orders, to "come home when he finished his task". Regulus was not happy to learn about Voldemort's cruel treatment on his elf.
Regulus later had Kreacher take him back to the Crystal Cave. Most noble wizards would had their House Elves to drink Voldemort's potion, instead Regulus drank the potion himself and ordered Kreacher to switch Slytherin's Locket with a fake locket and then go home and destroy the "real" locket, leaving Regulus to die in the cave. For the last few seconds Kreacher was there, he saw the Inferi's hands pulling Regulus into the water. Regulus also ordered Kreacher to never tell his family what had happened to him. Kreacher despaired, but never dared to defile his master's orders, so he left and never told the rest of the Blacks the real fate of Regulus. However, back at home Kreacher was unable to destroy the locket, despite his many attempts. This inability to complete an order of his master may have driven him to insanity, explaining his muttering and devotion to his mistress' painting.
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 10 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." Like the other house elves, Kreacher always spoke in third person, referring to himself.
Kreacher later redeemed himself because he told Harry he helped Regulus, to get the Horcrux locket and destroy it. Kreacher told Harry, Ron, and Hermione that 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. Hermione tried to hug Kreacher, but the latter was horrified to be touched by a muggleborn. Kreacher explained how the locket was later stolen by Mundungus Fletcher, who was then later caught by Dolores Umbridge as he was selling stolen objects, but managed to bribe Umbridge with the locket. 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. Harry and Ron finally listened to Hermione and spoke to Kreacher with well-mannered friendly behavior. Kreacher was surprised by Harry and Ron's sudden nice tones and praises for his work. What really moved Kreacher was Harry, who gave him Regulus' locket that Harry and Dumbledore had recovered, and promised him that he would finish Regulus' job. Ever since that moment Kreacher became obedient and nice to Harry and gladly accepted him as his master. He found Mundungus for Harry. Harry was shocked that Hermione and Dumbledore were right all this time, as he expected Kreacher to hate him anyway. He was forced to admit that indeed Sirius indirectly caused his own demise by making himself hated by the elf. At the begin Kreacher still found difficult to accept Ron and Hermione. But eventually he got used to it, and the athmosphere at Grimmauld Place became nicer and easier, with Kreacher loyal. Before all that, Harry and friends would only get poor dishes with lousy taste, with possibility of Kreacher spitting on it, and now it all the food was good and much more different and rich. Ron came to bless the elf for his good food.
After their mission at the Ministry of Magic, Corban Yaxley managed to follow Hermione at Grimmauld Place, which forced the trio to leave and camp in the countryside. Harry thought sadly about Kreacher waiting for them with the promised dinner. However, Kreacher went to Hogwarts and fought in the Battle of Hogwarts, leading the house-elves into battle. He survived.