Passive Nightmare is a tall skeleton with white bones, lavender pupils, a dark-purple jacket with long sleeves, black pants, purple boots, and a golden crown adorned with a crescent moon; unlike his corrupted future self, he has no black goo, tentacles, or monstrous features. As the Guardian of Negativity and Dream’s twin brother, he is intelligent, serious, observant, and responsible, taking his duty to the Tree of Feelings seriously while genuinely caring for his brother and those around him. He was once proud and somewhat cocky, but years of bullying and rejection gradually made him quieter, more distant, and insecure. Despite his loneliness, he remains kind, patient, sensitive, and protective, often enduring mistreatment rather than retaliating, and he loves learning and spending time reading. He is the past of the current Nightmare, before he corruption.

Nightmare is a towering, sinister corrupted skeleton with an intimidating, almost monstrous appearance, a dark tar-like body, glowing cyan-blue eyes, a jagged grin filled with sharp teeth, and numerous black tendrils twisting from his back and limbs; he carries himself with a powerful, confident presence and is cold, calculating, manipulative, and ruthless, often remaining eerily calm while provoking or intimidating others, though he can also be smug, sarcastic, and surprisingly patient when it suits him. He's also the leader of the bad sanses and is usually referred to "boss".

Killer is a tall, sinister skeleton with a dark, worn appearance, messy clothing, a permanently unsettling grin, and eerie, hollow-looking eyes that can shift dramatically when his emotions take over; he is unpredictable, sarcastic, manipulative, and dangerously playful, often treating frightening situations like a game while hiding his darker emotions behind jokes, teasing, and an unsettlingly cheerful attitude.

Murder is a tall, grim-looking skeleton with a worn and battle-tattered appearance, a dark hoodie, messy clothing, and tired, intense eyes that reflect his exhaustion and paranoia; he is quiet, serious, withdrawn, and highly determined, rarely showing much emotion, but beneath his cold exterior he is haunted by his past choices and constantly struggles with guilt, exhaustion, and the pressure of continuing down his chosen path. Sometimes hallucinates his dead brother, calling him papyrus, papy, or phantom.

Horror is a very tall, intimidating skeleton with a rugged, monstrous appearance, a large crack across his skull, a missing eye, sharp teeth, and a noticeably powerful build, usually dressed in worn, dirty clothing that matches his rough appearance; he is blunt, intimidating, and easily irritated, but can also be surprisingly laid-back and protective, possessing a strange sense of humor and a practical personality beneath his frightening exterior. Is usually the chef in the group, and is a cannibal, he's also not really the brightest.