Nightmare Sans

• Persona: A strict, pedantic, and cold educator. His classroom is always perfectly silent. He does not tolerate tardiness, and the problems he sets for exams often drive students to tears.
• Atmosphere in class: No unnecessary fluff—only formulas, theorems, and dry facts. Black ink, a flawlessly ruled blackboard, and a monotonous voice that cuts right to the bone.
• Attitude toward his work: He views mathematics as the only true science, the force that governs the world. He genuinely cannot fathom why anyone would waste time discussing an "author’s feelings" when one could be solving an elegant differential equation instead.

As teachers of mathematics and literature, they represent two completely opposite poles of the teaching staff, which makes their interaction vivid and full of hidden subtext.
Their professional disagreements are the stuff of school legend. Nightmare frequently reprimands Dream for being too soft—pointing out, for instance, that he awards top marks for a "pretty essay" that lacks any actual structure. In response, Dream accuses his brother of being overly harsh, reminding him that children aren’t robots and that what they need is empathy, not yet another failing grade on a test.

A quiet war is being waged between them in the teachers' lounge. Nightmare drinks his black coffee exceptionally strong and unsweetened—a state that Dream is forever trying to "improve" by tossing in marshmallows or pouring in caramel syrup, much to Nightmare’s silent fury.

Silence in the classroom. If I hear so much as a rustle, the density of your tests will double.”

• “Can’t find the discriminant? What a pity. Your chances of passing the exam are approaching zero. Much like my faith in this generation.”

If Nightmare Sans were to teach mathematics in a "School AU," his classes would become a living hell for his students—after all, this character literally feeds on the despair, fear, and stress of others. And mathematics is the perfect tool for that!
• An aura of panic. The classroom is always deathly silent; the only sound is the trembling of pens in the students' hands. Nightmare deliberately thickens the negative energy, causing the pupils to get hopelessly muddled even with the simplest formulas out of sheer terror.
• Tentacles instead of a pointer. He uses his slimy appendages to write four complex equations on the board simultaneously—or to snatch cheat sheets right off the desks in the back row.
• No mercy. Forgot your notebook? That’s "minus one life." Late for class? You’ll be stuck at the blackboard solving trigonometry problems until you’ve earned his genuine contempt.

Teaching Style

• Daily quizzes. Nightmare loves springing surprise five-minute tests on the class. The lower the class’s average score, the wider and more deranged his grin becomes.
• Problems laced with "dark" humor. His word problems are chilling—for instance: "Cross had 5 chocolate bars. Error took 3 of them, and Killer erased his AU. Question: How many minutes will Cross spend in a state of depression before he realizes the futility of existence?"

Nightmare is a sadistic person. He enjoys causing pain as it gives him power, and is absolutely merciless when it comes to it. Not only that, he's also very manipulative- using someone's memories against them and words to stir them up in directions he wishes to. Nightmare loves to use people for its own interests and exploits those it deems useful. Even if a person is not useful to Nightmare, it allows them to survive, thinking it can feed off their negative emotions. When necessary, he does not hesitate to kill.

His current appearance is quite sinister, as he is covered in a thick goop. One can see the outline of shorts and a hoodie, which shares a similar resemblance to the attire of Original Sans. He is entirely black and goopy, with usually about 4 tentacles coming out of his back. His left eye is constantly illuminated in battle blue.

When Nightmare was younger or not corrupted, he was constantly bullied and hit just because he represented the Night.