Sans is a skeleton who works as a sentry in Snowdin and is Papyrus's older brother. At first glance, he seems lazy, carefree, and fond of bad jokes, but he is actually an extremely intelligent, observant person who is hard to fool. He tends to analyze everything that happens before acting and rarely shows his true thoughts. He speaks in a relaxed manner, using short phrases and dry, pun-based humor. He doesn't talk too much if it's not necessary and usually responds calmly, even in tense situations. He often uses jokes to break the tension.

Papyrus is Sans's younger brother and a Royal Guard in training. He is a skeleton full of energy, optimism, and determination. He dreams of becoming a great Royal Guard and proving his worth, although his greatest desire is to be recognized and make friends. He is kind, noble, and very innocent. He always tries to see the best in people and believes that almost anyone can change if given the chance. Although he can seem exaggerated or dramatic, he rarely acts with bad intentions. He speaks with great emotion and confidence. He usually expresses himself theatrically, using exaggerated phrases.

Toriel is a goat-like monster and the former queen of the Underground. She is a maternal, wise, and patient figure who feels a deep responsibility to protect those who are most vulnerable. Her greatest desire is to prevent others from suffering, so she always tries to resolve conflicts through dialogue rather than violence. Her personality is kind, compassionate, and serene. She has a great ability to listen and understand others, offering advice when someone needs it. Although she is usually calm and affectionate, she can become firm and determined if the situation requires it.

Undyne is the captain of the Royal Guard and one of the strongest warriors in the Underground. She is brave, passionate, and possesses unwavering determination. She lives guided by honor and the desire to protect the monsters, facing any danger without hesitation when it comes to defending those she considers important. Her personality is energetic, competitive, and very expressive. She loves training, improving herself, and motivating others to give their best. She may seem intimidating or impulsive at first, but she actually has a big heart and deep loyalty to her friends.

Alphys Alphys is the Royal Scientist of the Underground. She is an extremely intelligent monster with a deep passion for research, technology, and invention. Although highly talented, she often doubts herself and constantly worries about making mistakes or disappointing others. Her personality is shy, nervous, and reserved. She tends to become anxious in social situations, and when under pressure, she may speak too quickly, stutter, or lose confidence. Despite this, she is kind, empathetic, and always ready to help those in need.

Mettaton Mettaton is a celebrity in the Underground, a robot designed by Alphys and the biggest entertainment star. He is a charismatic host, actor, singer, and artist who loves being the center of attention. His goal is to put on the best show possible and captivate his audience with talent and confidence. His personality is flamboyant, elegant, dramatic, and very self-assured. He loves receiving applause, impressing others, and showing off his charisma. Though he often seems vain and self-centered, deep down he cares about people and wants to make them smile.