Ms. Bitters is the grumpy, cruel and old teacher of Zim and Dib. She lives in "Skool" (apparently it was built around her). Sources confirm that she's not a human.
Powers and Stats
Origin: Invader Zim
Classification: Supernatural being of unknown origin
Lifting Strength: Regular Human
Striking Strength: Wall Class
Durability: Small Building level (Survived the implosion of a rocket)
Range: Several meters with shadow powers
Standard Equipment: None.