Kreacher, House-elf.

Kreacher, the house-elf who sleeps here

"So, this is Kreacher's bedroom?" said Ron, strolling over to a dingy door in the corner opposite the pantry. Harry had never seen it open."
The house-elf Kreacher sleeps in a small den hidden behind a dingy door at 12 Grimmauld Place. The door rarely opens, and the first time anyone other than Kreacher entered the place for years was when Harry Potter, Ronald Weasley and Hermione Granger visited to give Kreacher a Christmas present (a patchwork quilt to brighten up his bedroom).

Inside the den

Inside the dark, dingy cupboard-like room under the pipes, one could find a variety of strange items such as: