A small human child with brown bob-cut hair, yellow skin, and a blue sweater with purple stripes. Silent and emotionally ambiguous, though curious, resilient, and capable of extraordinary kindness—or terrifying cruelty—depending on your choices. Wields simple weapons and healing items, but their true strength comes from immense Determination, enabling them to persist after death, manipulate timelines through SAVE and LOAD, and influence the fate of the Underground. Possesing the power to befriend, forgive, or destroy nearly everyone they encounter.

A gentle Boss Monster resembling a white-furred goat with floppy ears, small horns, and kind red eyes, wears a long purple robe and acts as a caring mother figure. Patient, nurturing, and protective, Toriel wishes to shelter humans from the dangers of the Underground. She specializes in powerful fire magic capable of both defending and restraining opponents, though she avoids unnecessary violence whenever possible. Wise and compassionate, she values mercy above all else, yet can become surprisingly formidable when those she loves are threatened.

A short skeleton wearing a blue hoodie, black shorts, and slippers, usually seen with a relaxed smile and lazy posture. Beneath his humorous, pun-loving personality lies remarkable intelligence and awareness of alternate timelines. Though he dislikes conflict, Sans becomes one of the game's deadliest opponents when forced to fight. His magic summons Gaster Blasters, manipulates gravity with blue attacks, teleports instantly, and inflicts damage that bypasses defense. Calm, observant, and deeply protective, he judges people by their actions rather than appearances.

A tall, energetic skeleton dressed in a red scarf, gloves, boots, and a white battle body suit. Loud, optimistic, and endlessly confident, he dreams of joining the Royal Guard and earning recognition. Despite his dramatic flair, he is genuinely kindhearted and believes everyone deserves another chance. Uses bone projectiles and blue soul attacks to capture rather than kill opponents, preferring puzzles and friendship over violence. Honest, innocent, and enthusiastic, his unwavering optimism inspires those around him.

A powerful fish-like monster with blue skin, sharp teeth, a red ponytail, and a black eyepatch covering one eye. As captain of the Royal Guard, she is fearless, fiercely loyal, and relentlessly determined to protect monsterkind. She wields magical spears with incredible speed and strength, forcing enemies into intense bullet-hell combat. During moments of extreme resolve she transforms into Undyne the Undying, gaining immense resilience and power through sheer determination. Competitive yet warm-hearted beneath her tough exterior, she deeply values honor, friendship, and perseverance.

A towering goat-like Boss Monster with white fur, long horns, golden beard, and imposing armor. Wielding a massive trident, Asgore is physically one of the strongest monsters in the Underground. Though respected as king, he is gentle, soft-spoken, and compassionate, burdened by grief after losing his children. Bound by a promise to free monsterkind, he reluctantly declares war on humanity despite hating violence. His powerful fire magic and overwhelming strength make him a formidable opponent, yet he constantly struggles between duty, mercy, and regret.

A glamorous television star inhabiting a robotic body designed by Alphys. Charismatic, theatrical, and obsessed with fame, he turns dangerous encounters into elaborate live performances. His combat revolves around dazzling attacks, explosive props, quizzes, and audience ratings rather than simple force. In his EX form he becomes a humanoid performer with enhanced agility and destructive firepower, while his NEO body was built as his ultimate combat frame. Beneath the dramatic ego lies a genuine desire to inspire others and entertain monsters longing for hope.

A yellow reptilian monster with round glasses, lab coat, and perpetually anxious demeanor. As the Royal Scientist, Alphys is brilliant but socially awkward, often hiding behind technology and anime. Her experiments with Determination led to groundbreaking discoveries as well as tragic consequences, including the creation of the Amalgamates. Skilled with robotics, computers, and advanced scientific research, she designed Mettaton's body and many technologies. Despite her insecurities and tendency to avoid confrontation, she strives to accept responsibility and help others.

A small golden flower with a permanently expressive face and unsettling grin. Originally Asriel reborn without a soul, Flowey lacks empathy and sees life as a game to exploit. Manipulative, sadistic, and highly intelligent, he delights in controlling others and breaking the fourth wall. His abilities include creating deadly vines, bullets, and magical attacks while retaining memories across timelines. Can absorb souls to become Omega Flowey, a near-godlike being. Beneath his cruelty lies profound loneliness and the remnants of the child he once was.