She seems to possess larger eyes than the named members of the flock, and it appears that she doesn't have upper eyelids.
Shaun the Sheep
Shaun the Sheep: The Farmer's Llamas
Shaun the Sheep Movie
A Shaun the Sheep Movie: Farmageddon
Hazel's Voice Actress Over here Also Voiced Perfect Peter From CITV's (Boomerang and Comedy Central In US) Horrid Henry