Sans and Papyrus

Frisk is the protagonist, the player's avatar. In the true ending, his name is revealed.
Flowey is initially a sadistic flower, but is later revealed to be the reincarnation of Asriel (the Prince of the Underground).
Toriel is a female monster resembling an anthropomorphic goat. She is the kind guardian of the Ruins and tries to protect the hero.
Sans is a skeleton, Papyrus's brother. He is known for his lazy nature, witty jokes, and hidden sense of justice. He is the final boss in the Genocide route.
Papyrus is another skeleton who dreams of joining the Royal Guard. He's energetic, yet kind and naive. Undyne is a fish woman and the leader of the Royal Guard. Determined and driven, she transforms into a stronger form (Undyne the Invincible) in the Genocide route.
Alphys is a lizard scientist in the service of the king. She created the robot Mettaton with a soul. It is later revealed that she was secretly framing the hero.
Mettaton is a robotic TV presenter created by Alphys. Has several forms (Mettaton, Mettaton EX, Mettaton NEO).
Asriel Dreemurr is the prince of the Underground, the biological child of Toriel and Asgore, and Chara's half-brother. In the True Pacifist Route's true ending, he appears as a powerful being.
Chara is the first human to fall into the Dungeon. Her soul plays a significant role, especially in the Genocide route.

Appearance: A tall skeleton with an anatomically imprecise cranial structure (more vertical and geometric than his brother's). His mouth is almost always frozen in a smile, and his pupil-less eyes resemble vertical slits. Clothing: He wears a "battle body"—a homemade costume he created for a costume party. It consists of an orange-red cape, a white chestplate with gold trim at the bottom, blue underwear with a gold belt, red gloves, and red shoes. On dates, he may change into a "secret style": a backwards baseball cap, a T-shirt with the slogan "Cool Dude," short button-downs, striped gloves, knee-length socks, and sneakers. Personality: Hot-tempered, charismatic, self-confident, and sometimes selfish, calling himself "The Great Papyrus." At the same time, he is hardworking, kind, optimistic, and brave. He can be forgetful and unobservant at times, but can be cunning (for example, he used reverse psychology to trick Undyne into befriending the protagonist). Papyrus truly believes in the kindness of people and believes that anyone can become great if they just try. He longed to become a royal guard to gain fame and friends. To this end, he patrolled Snowdin Forest and created puzzles in hopes of capturing people. Papyrus enjoys puzzles and riddles, and is also a keen figurine collector. He loves cooking spaghetti, although many characters consider his cooking less than stellar. In battle, Papyrus uses bone manipulation, soul magic, and gravity manipulation (coloring the enemy's soul blue, causing it to move more slowly).

Sans is a rather short and "big-boned" skeleton. He is always recognizable by his wide, toothy grin. He wears an unbuttoned blue jacket (or sweatshirt) with a gray hood, revealing a white T-shirt underneath, black shorts with white stripes, and slippers (in his official Steam card, they're sneakers). His black eye sockets contain white pupils that disappear when he's serious or angry. When using telekinesis, the iris in his left eye appears, flashing blue and yellow (the colors of patience and justice), while his right pupil disappears. Sans has a casual, relaxed demeanor. He loves to joke, and his puns (often skeletal-themed) greatly irritate Papyrus. At the same time, Sans is kind and believes in the good in others: he believes that people (or the protagonist in the game) do bad things because of circumstances, not out of malice. He doesn't like making promises, but when he does, he tries to keep them. For example, he promised Toriel to protect fallen people. However, Sans has a hidden side. He is often called lazy: he does just enough work to keep Undyne from firing him. But this laziness is a consequence of his knowledge: he understands that the player can save and load the game, meaning resetting progress. Because of this, Sans has developed a nihilistic worldview: since everything can be undone with a single reset, then effort seems pointless. He denies this. Sans has a scientific background: he has a textbook on quantum physics, a secret workshop, and is interested in timeline research. Flowey even noted that Sans was the cause of many of his resets. Sans is also left-handed.