The Grimalkin is a malk, cat-like faerie acting as a servant to Mab. She uses him as an interpreter, with Grimalkin speaking to other people in her stead while she remains silent. He first appears in Summer Knight.
In Summer Knight, Harry Dresden describes his voices as sounding like a tortured cat given the gift of speech. He says he's been called many names and trod many paths: hunter, watcher and guide. His glowing pawprints had the vague shape of a feline paw, but too spread out and too thin to be an actual cat's pawprint. 
The Grimalkin is described as very large and muscled, weighting about sixty to seventy pounds. He has a dark grey fur with a rippling black pattern. His eyes are a yellow-green color and very large, and have a quality about them that reveals a great intelligence not found in a common animal.
In the seriesEdit
In Summer Knight, Harry Dresden is following Elidee through Undertown, seeking Maeve's court. He comes across Grimalkin who says he was bidden by the Cold Lady, Maeve, to guide her mother's emissary to her court. Grimalkin left pawprints that had a dull greenish glow on the ground.
He is seen twice again in Small Favor, where he acts as Mab's voice. The first time is when she gives him the task of finding Marcone who was abducted by the Denarians. At the end, Mab appears in the hospital chapel right after Uriel left. She has Grimalkin on her lap still speaking for her.