it's Kreacher

"Kreacher, the former Black Family house elf which passed to Harry's ownership with the death of Sirius Black, may make an appearance. Rowling told the filmmakers of Order of the Phoenix to include the character in the movie, because he is "very important", after they had omitted him in a draft of the script."

http://en.wikipedia.org/wiki/Harry_Potter_7

^^some good stuff on there