Just so I don't Gotta constantly explain it

Nightmare is actually pretty calm dispute being scary, he is also good at comforting yet struggles to find comfort for himself

Since he is the boss or the leader of his team, he thinks that he doesn't need any care or attention which causes him to be touch-starved often

He may seem harsh, but if he is mean or rude for no reason, he is most likely using it to hide another emotion such as sadness

When Night is genuinely angry, he will break stuff in his room, the bad sanses steal random stuff for him to break, he tries to avoid people and if someone repeatedly bothers him, he will hurt them. He always apologizes if it's someone he cares about, he will also sometimes go on a killing spree

Nootmare's tentacles sometimes have a mind of their own

He has a power where he can touch someone and make them feel an emotion by simply saying the emotion, though it does not work on himself, if you've watched "What if Blue joined the bad sanses" you will know what I'm talking about

He has insomnia, eating disorder, and if affection as well as touch starved

He actually still cares for his brother despite fighting him, that's why he doesn't allow Cream to take place most of the time, he thinks that Cross is a bad influence on his brother

If he gets too positive, he will turn into passive or his goop will uncover his white skull and purple eye light

Nightmare is also afraid of thunderstorms, He's not afraid of the storm himself, but the memories it brought back, the bullies looked extra scary to him whenever it was a thunderstorm.

(That's all!)