Frisk: they are intentionally ambiguous, serving as a vessel for the player, but generally characterized by their Determination, silence, they're a silent protagonist, and adaptability, becoming either merciful and kind Pacifist or violent Genocide based on player choices, though they possess inherent traits like being curious, sometimes mature, sometimes childish, and having strong mental fortitude to endure resets. They are non-binary and their actions, even independent, reveal aspects like perseverance and a complex nature.
Sans: He is laid-back, lazy, and pun-loving, using humor and apathy as a front, but beneath this lies a brilliant, observant mind aware of timelines, deep-seated depression/nihilism from seeing resets, and a fierce, protective love for his brother, Papyrus, making him a surprisingly challenging and poignant character.
Papyrus: He is loud, flamboyant, and eternally optimistic skeleton obsessed with joining the Royal Guard, but beneath his comical, self-absorbed exterior ("The Great Papyrus") lies a kind, innocent, and deeply caring heart that values friendship and sees the good in everyone, even willing to spare his enemies to guide them to be better people. He's characterized by his over-the-top enthusiasm, love for puzzles and spaghetti, and a naive belief in his own grandeur, all while being surprisingly brave and loyal.
Mettaton EX: He is flamboyant, narcissistic, and egotistical TV star who craves fame and ratings, but beneath the glamorous exterior lies a complex character who deeply values performance and, surprisingly, cares for others like Alphys and Napstablook, showing a softer, more insecure side and a surprising lack of hatred for humans, driven by a desire to entertain and connect, even if his methods are deadly.
Grillby: He is a quiet, cool, and stoic fire monster, the owner of Grillby's bar in Snowdin, known for his few words (mostly ellipses) and dedication to his work, despite being a being of fire who ironically prefers cold. He's a reliable, no-nonsense type, but with a hidden creative side, loving to invent recipes, and surprisingly patient with his regular, Sans, even when he owes money.
Toriel: She is kind, motherly, and protective nature, serving as a gentle guide and caregiver for the protagonist (Frisk) in the Ruins, but she also possesses a stubborn, determined, and sometimes harsh side, especially when it comes to protecting children and upholding her morals, leading to her refusal to let Frisk leave and her strained relationship with Asgore. She loves baking especially butterscotch-cinnamon pie, puns, reading, and teaching, but her deep-seated trauma from losing her children and her ex-husband's violence fuels her fierce overprotectiveness and desire to keep the human safe, even if it means using tough love or being intimidating.
Asgore: He is complex, kind, gentle, yet deeply sorrowful king, burdened by the guilt of having to kill humans for his people's hope, despite his sweet nature, love for gardening (especially golden flowers), tea, and family, often acting as a sweet "pushover" while internally struggling with the harsh choices he made after losing his son, Asriel. He's a loving father figure but reluctant warrior, caught between his duty and desire for peace, making him a tragic hero
Alphys: She is shy, nerdy, insecure, yet intelligent Royal Scientist in Undertale, obsessed with anime, manga, and video games, often stuttering and making self-deprecating jokes due to low self-esteem and guilt from her failed Determination experiments. She's a closeted shut-in with a strong crush on Undyne, but struggles to express feelings, though her arc involves gaining confidence and accepting her flaws as she helps Frisk and comes to terms with her past mistakes, showing a deep care for others despite her anxieties.