Kreacher cheered when Sirius Black went to prison. Two years later when his mistress died, and he no longer had her telling him how awful her oldest son was, Kreacher remembered that once, his dear Master Regulus was fond of Sirius Black. The horcrux had not been acting on him for as long, and he was able to escape, briefly, from the malice which wrapped his soul.
And Sirius Black was a Black, and should not be thrown in Azkaban without a trial.
Kreacher went to visit Sirius Black. It should not have been possible, but then, Kreacher had ingested a potion made under the sort of extraordinary circumstances that the aura of the dementors did not block his apparation.
"Master Sirius can't have done what they said?" said Kreacher, resisting the urge to abuse the already ragged prisoner.
"I didn't," said Sirius. "It was that damned rat, and he isn't dead. What's your game, Kreacher?"
"Master Regulus left a task and Kreacher cannot do it," said Kreacher, twisting his long fingers together. "A bad thing made by the Dark Lord; but Kreacher promised Master Regulus not to tell his family about it. But Kreacher needs Master Sirius to know when he is free."
"They threw away the key, Kreacher," said Sirius.
"No trial for Lord Black! Jumped up tainted blood ministers, going against the ancient and noble house of Black," said Kreacher. "No class. Kreacher needs to know about the rat." He considered. "Kreacher will be back."
Kreacher was back rapidly with pork chops, mashed potato, peas, sprouts, and roast onion.
It is a measure of how hungry Sirius was that he didn't even whine about the sprouts being included.
Sirius told Kreacher all about how they were animagi to support their werewolf friend, and was clever enough to mention that Remus was pure-blooded and that Greyback was not.
Kreacher provided a good meal every other day to his master, and went looking.
Molly Weasley was more inclined to look after a skinny, ageing house-elf than let it work for her when the poor creature turned up, claiming line death freedom and wanting a family. Kreacher punished himself severely for this lie to a pure-blood, albeit blood-traitor family, in making death-by-chocolate for his master, whose enjoyment was a distinct punishment to him.
It did not take much for the elf to seize the rat, and pop to the DMLE with him, appearing in front of the new head of the DMLE, Amelia Bones.
"Kreacher has the real traitor to the Potters," he squeaked.
The Bones family was a good traditional family, she would do very well, thought Kreacher, as Lady Black. Rescuing his master from Durance Vile was a good way for her to be seen in a positive light.
House elves talk amongst themselves, and Mister Bartimeus crouch was easy to blackmail. It made him free Winky for speaking to freely, but Kreacher had been in charge of hiring, and the elf could bond to their master fully later; meantime she could bond through him. It was about time he had elflings, and Winky was obliging and trainable if not an ideal mate.
And once his master was free, Kreacher told Winky about the horcrux and she sobbed all over Lord Black and Missy Amelia.
The DMLE was now actively involved in horcrux hunting; and Kreacher was involved in hunting for Harry Potter, who could at least be reared as a pure blood even if he was a half-blood.