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Ms. Bitters is Dib's and Zim's cranky teacher. She's quite old, and seems to be capable of moving and hovering like a shadow. Supposedly she has been teaching for a very long time, as "the school was built around her".
MsBittersIZ

She is thought to be alien, though some disagree. She also appears susceptible to the sunlight, as she states she cannot be in the sun for long.