This question already has an answer here:
Kreacher is the house-elf of Grimmauld Place. From the death of the last black inhabitants, he was left alone until Sirius moved in. Then his ownership passed on to Harry.
Harry had him work at Hogwarts until DH when he had him back to Grimmauld Place. Some trust started to build between them, but after the Ministry of Magic chaotic escape, the house became accessible to Death Eathers (or at least presumably). Harry even thought about Kreacher for some time, and how he would be saddened by their disappearance and the invasion of the Death Eaters.
Nevertheless, we never get to know what happens to him afterward, IIRC. Is there any information about him after Harry, Ron and Hermione broke into the Ministry of Magic?