Appearance

Hair: Black and greasy, at the height of the shoulders.
Eyes: Black.
Complexion: sallow also described as white or pale.
Distinguishing features: long hooked nose, dark mark on his left forearm, not always visible.
Clothes: Black robes.

Descriptions from the books:

"A teacher with greasy black hair, hooked nose and sallow skin".
"His eyes were as black as Hagrid, but did not emit the same heat. They were cold and empty and made you think of dark tunnels".
"Uneven, yellowish teeth".