Nightmare Bitters is the doppelgänger of Ms. Bitters and the leader of the Halloweenies in Invader Zim. She is even more terrifying than the real Ms. Bitters.
Nightmare Bitters is the leader of the Halloweenies who are very demonic people in the Nightmare Realm from Dib's head. Bitters sought to take Dib and go to the real world. However, her allies got Zim instead. She sent the Halloweenies to get Dib and perform the operation so that she can lead her people to the real world. As she got into the real world, it was Halloween and she saw an obese GIR. She yelled in fright and came back to the Nightmare Realm.
- "This is the wrong one! You have failed me!!"
- "You will be condemned to the Realm of Eternal Screaming and Restlessness!"
- "You just have to accept it! Here's your ticket."
- "Let the head... thingy... start!"
- "At last! A new perfect world for me to... Huh?!"
- She is voiced by Rodger Bumpass.
- She is one of the darkest Invader Zim villains.