Ms. Bitters Is Not Human

Ms. Bitters
Ms. Bitters is Zim and Dib's teacher. She has never been married (otherwise she would be known as Mrs. or Mz. Bitters) and is known for hating children and saying very negative things. She is very careless about her students and almost always finds a way to bring them down. Through out of all the students that she hates, she seems to hate Zita the least.

Attributes of Ms. Bitters
Ms. Bitters has exhibited a number of odd behaviors, abilities, and stated weaknesses such as;
 * Clawed Hands
 * Covered in Cochroaches during early episodes
 * Hissing (like a roach or snake)
 * Levitation
 * Inhuman Elongation aka Seprentine Movements such a slithering
 * Materializing Out of Shadows
 * Weakness to Sunlight (Episode "The Voting of the Doomed")

Ms. Bitters on her Past

 * She stated she was once a fairy princess till something went wrong. We see a clip of her as a fairy getting zapped by a Bug Zapper. (Episode "Halloween Spectacular of Spooky Doom")
 * She stated she wanted to be an astronaut until her ship imploded. We see a clip of a young girl who looks a lot like Zeta in a rocket ship that implodes. (Episode "Career Day")

Species
It is unknown what Ms. Bitters actually is. Some say that she is a vampire or demon because of the episode The Voting of the Doomed, when she mentions that she can only survive in the sun for so long. It has also been known that she makes inhuman movements such as floating, slithering, and forming out of shadows.

Former Tallest
Some believe that because of her ability to float and her height, that Ms. Bitters is Irken and used to be one of the Tallests. One theory is that when two more Irken's were found to be taller than Ms. Bitters, they sent her on a spaceship that was targeting a random planet to land on, which happened to be earth. She had to make a disguise and form a "normal" life on earth (and obviously did a better job than Zim). Another theory is that Mrs. Bitters simply retired or there had been an election for a new tallest. The main wrench in this theory is the episode "Lice" where it's revealed that Zim's skin (Irken skin) kills Lice, while Ms. Bitters avoids having her skin tested for immunity to lice it's revealed at the end of the episode that she has caught Lice. The fact that she intentionally makes an effort not to be tested for immunity does however support the theory that she's not Human or has something to hide that could be uncovered on a genetic level.

Insectoid
In Issue 15 of the Invader Zim comic series her students began to make some theories regarding her origin during her absence. Although many of the children's theories seemed plausible, the most likely was Gretchen's. She believed that Ms. Bitters was an extremely intelligent insect queen who spread a message to her servants to study human strengths and weaknesses. They pursued the second part of Ms. Bitters' plan, and started a war with the Humans. They lost a great deal of bugs but still gained the upper hand, and the humans retreated. Ms. Bitters and her bugs, now low in numbers, burrowed under the Skool, and now they wait patiently in hiding for the right time to strike again. If Ms. Bitters was an insect queen this would explain her odd tendency to be covered in cockroaches, her prolonged immunity of lice, and the insect-like appearance of Nightmare Bitters, who also happened to be the leader of all the other doppelgängers. A Ms. Bitters bug was also spotted by Poonchy, smirking at him menacingly as he watched in horror and awe. Though one has to wonder what motive she'd have for contacting lice removal specialist- unless her people and lice kind are rivals. In "Halloween Spectacular of Spooky Doom" she states that she was once a fairy princess until something went horribly wrong, at which point we see her as a fairy flying into a bug zapper.