Sans — Appearance: A short, stout, and permanently grinning skeleton with hollow black eye sockets that can flash with a glowing blue/yellow iris. Attire: An unzipped blue hoodie over a white t-shirt, black shorts with white stripes, and pink slippers. Persona: Relaxed, witty, and nihilistic on the surface; secretly highly observant, deeply depressed, and a deadly protector of reality's balance.

Papyrus — Appearance: A tall, slender skeleton with large, expressive eye sockets and straight, blocky teeth. Attire: A custom costume ("battle body") consisting of a white chest plate with red trim, a short red scarf/cape, red gloves, and matching red boots. Personality: Energetic, flamboyant, and deeply optimistic; despite his desire to join the Royal Guard, he is incredibly kind-hearted and struggles to harm anyone.

Undyne — Appearance: An anthropomorphic blue-scaled fish monster with a long red ponytail, sharp yellow teeth, and a black patch over her left eye. Attire: Varies from full, polished silver plate armor to a casual black tank top, blue jeans, and boots. Personality: Intense, passionate, and fiercely patriotic, driven by unwavering determination and a deep love for her friends and the monster race.

Alphys — Appearance: A short, reptilian monster with smooth yellow scales, a prominent overbite, and three blunt spikes on the back of her head. Attire: A standard, oversized white lab coat worn over a simple polka-dot dress, paired with round, wire-rimmed glasses. Personality: Socially anxious, insecure, and stuttering, with a brilliant scientific mind heavily contrasted by a deep obsession with anime and human pop culture.

Mettaton — Appearance: Initially a boxy, silver rectangular robot with a single grid-like screen interface; in his "EX" form, he becomes a sleek, humanoid android with long black hair. Attire (EX Form): A pink chest piece, a black waist module, shoulder pads, and dramatic long-heeled pink boots. Personality: Extravagant, vain, and show-stoppingly charismatic, living entirely for the spotlight and the adoration of his fans.

Asriel Dreemurr — Appearance: As a child, a small, hornless white goat-like monster resembling a young Toriel or Asgore; in his "God of Hyperdeath" form, he becomes a towering, horned figure with black facial markings. Attire: A green-and-yellow striped long-sleeved shirt as a child; an elaborate dark robe with the Delta Rune on the chest in his final form. Personality: Originally a timid, sensitive, and fiercely loyal child, his true self is deeply emotional and desperate to fix past mistakes after spending lifetimes as the emotionless Flowey.

Asgore Dreemurr — appearance: A large, powerful, anthropomorphic goat-like monster with thick white fur, a broad blonde beard and mustache, and long curved horns. Attire: Heavy golden armor with a purple cape fastened by a Delta Rune clasp, though he frequently wears a simple floral apron when gardening. Personality: Soft-spoken, remorseful, and deeply gentle; he acts as a reluctant warrior driven to commit tragic acts only out of duty to his trapped people.

Toriel — Appearance: A maternal, anthropomorphic goat-like monster with drooping ears, short horns, white fur, and small visible fangs. Attire: A long, royal blue and purple gown emblazoned with the "Delta Rune" emblem on the chest. Personality: Deeply protective, warm, and maternal, though she can be stubborn and fiercely defensive of those she cares about.

Flowey — Appearance: A sentient golden flower featuring six bright yellow petals around a white center, capable of shifting into terrifying, distorted facial expressions. Attire: None (purely biological plant structure). Personality: Cruel, manipulative, and nihilistic, operating under a strict philosophy of "kill or be killed" due to an absence of a soul.

Frisk — Appearance: A young, human child with a yellow complexion, a short brown bob haircut, and a neutral, squinting expression. Attire: A blue, long-sleeved shirt with two purple stripes, plain blue pants, and brown shoes. Personality: Quiet, determined, and highly expressive through their actions; their moral alignment shifts entirely based on Guest’s choice.