Papyrus
Papyrus is a loud and flamboyant skeleton who presents a confident, charismatic image of himself. He tends to work hard, and despite his brash personality, he is a very kind skeleton at heart. He is extremely optimistic and innocent, as shown on a Genocide Run when he continues to believe the player is good at heart, even as the player attacks and ultimately kills him. Papyrus is occasionally rather oblivious and nonobservant, such as when he tries to rub different kinds of creams and "sauces" behind his ears, only to realize that he doesn't have any ears. He can be witty at times, especially when he used reverse psychology to trick Undyne into making friends with the player.
Toriel
Toriel is a very kind, motherly monster who genuinely cares for the protagonist and their well-being. While ultimately a benign figure, she does have a harder side to her personality, as seen by her refusal to allow Frisk to leave the Ruins and by her cold treatment of Asgore. In addition, she has something of a mischievous streak, evidenced by her interactions with Sans. Toriel enjoys cooking. Many of her recipes use snails as a primary ingredient, as her favorite food is snail pie. She is also able to bake other things quite well though, as shown when she bakes a cinnamon-butterscotch pie for her and the player (excluding Hard Mode, in which she makes a snail pie in due to lack of ingredients.). Toriel sprite Toriel's in-game sprite. She has always wanted to be a teacher. By the time the protagonist reaches her house, she states that she has already prepared a curriculum for their education. This is also illustrated in the game's credits, as she is shown standing outside a school, and in certain Neutral Endings, in which she starts a school.
Flowey
Flowey wears a mask of friendliness and politeness. He has a habit of greeting you with "Howdy!", calling himself "your best friend", and, like his father, appending polite words such as "gosh" and "golly" to the ends of his sentences. Whenever dropping his facade and gloating, he also has a habit of calling you an "idiot". His central philosophy is that of Chara: that the meaning of the world is: "Kill or be killed." Flowey is initially kind and friendly, but as the first scene with him continues, depending on how your first battle with him goes he can slowly become more evil, or immediately reveal his kill or be killed attitude to the player, afterwards attempting to kill them. He is stopped by Toriel, but reappears at the exit of the ruins, commenting on the player's actions up to that point. As shown throughout the game, Flowey has a generally malevolent and cruel disposition, regularly berating a pacifist player for ignoring his kill-or-be-killed advice and generally talking about his desire to destroy the world. He is a cunning planner and subtle if the circumstances require it, even manipulating the player into bringing him as many monster souls as they can find in the neutral ending, being able to retain his memories between SAVEs (having once had the ability to do so himself). He also possesses a disturbing sense of humor, as shown in his boss fight, in which he turns into what is called "Photoshop Flowey". For all of his cruelty and generally evil actions, however, it is hinted he genuinely regards the player as something of a friend. As revealed in the Pacifist Ending and Genocide Ending, he lacks a SOUL, and with it, the ability to feel love or most other emotions-his evil may be an attempt by him to feel anything at all, even self-loathing.
Mettaton
Mettaton is a confident and charming TV host, that loves drama, action, and violence. He lives for his ratings and loves performing. He shows a rather shallow appreciation for existence at times, as referenced in various neutral endings. Beyond this, however, he sincerely cares for humanity and monster-kind, even mentioning Alphys specifically on multiple occasions.
Sans
Sans is shown to be very laid-back, sleeping on the job as often as he takes breaks. He enjoys making bad puns relating to skeletons, to the annoyance of his brother. He is kind and reassuring, but becomes eerily serious at abrupt moments (particularly when angry), demonstrating that his casual nature is perhaps a guise. He is also very observant, and his lines and actions change the most through the path and the way of playing the game. He knows the number of times he had defeated the player (in Genocide Route). He tends to say he hates to make promises, this is likely a result of having to harm a human, as he promised Toriel he would protect humans. His laziness is noted during the final fight in the genocide route - he is unsure if being somewhat aware of the timeline resetting has broken his spirit, knowing that everything he does will mean nothing... or if he's just using it as an excuse to slack off.
Asriel
According to audio logs found in the True Laboratory, Asriel was "a bit of a crybaby", somewhat cowardly and docile, and a kind and loving child who loved jokes and pranks. Unfortunately, a lab experiment restores him to life without his soul, turning him into the heartless Flowey. Left only with cunning and determination, Asriel eventually starts tormenting the monsters around him for sports, as he is unable to take joy in seeing even his parents happy. Growing bored of this, he sets sights on the human Frisk, who reminds him of Chara - the Fallen Human being his sibling and best friend, and the only being who understood him. Remembering his own loneliness, Asriel seeks to vicariously trap Frisk in an endless time loop to toy with them. As Asriel absorbs the monster souls of the entire Underground, Asriel temporarily regains his ability to feel emotions. Finally, he apologizes for his actions, and even despite the barrier being opened, he remains underground as he will soon turn back into a soulless golden flower.
Undyne
Undyne has an intensely passionate personality. She is strong-willed and confident as a result of her training under King Asgore, and she is unmerciful to her enemies. She over-zealously believes in Asgore Dreemurr and his plan to capture seven human souls in order to break the barrier and escape the Underground. She loves to teach, having mentored both Shyren and Papyrus in various skills, as well as instructing the player in how Green Mode works. She has a competitive spirit, but is ultimately close with those that she considers friends. She has deep-seated respect for Asgore, and romantic feelings for Alphys who has warped her perception of human history and culture due to her only exposure through the Eight Humans and anime. She is the only monster known to possess Determination naturally aside from boss monsters, though in much smaller amounts that allow for their souls to persist after death.
Asgore
Asgore sprite Asgore's in-game sprite. He is depicted as a kindly and easygoing family man; this is shown through his dialogue with Frisk during their encounter in the Neutral and the True Pacifist Route, as he attempts to be as kind as possible despite knowing that he will ultimately need to take their soul, going so far as to make small talk. He is an exceptional warrior, but is deeply uncomfortable with the act of killing. He enjoys gardening in his free time. Most other monsters in the Underground consider Asgore to be an excellent king, if bad at naming things. Toriel, however, criticizes him as an indecisive and weak ruler, preferring to keep the monsters trapped underground waiting to be saved rather than going out and taking the souls he needed to break the Barrier. All that has happened to him prior to the events of Undertale has left him very depressed. During Undertale, he has several difficult decisions to make, which he disagrees with morally, but feels compelled to enact. This burdens him heavily. By the time the player fights him, he is aware of the horrific things he did to try and save his people, and simply hangs his head in shame while he fights.
Alphys
Alphys is a nerdy, shy, recluse that takes a fondness to anime and comic books. She stutters often, and appears to have developed a large fascination with the main character after having watched them through the cameras spread throughout the Underground. She has romantic feelings for Undyne, which she believed to have been unrequited, although this proves to be untrue in the True Pacifist Route. When the answer of "Asgore" is given in Mettaton's quiz question asking who she has a crush on, she displays a similar reaction as to the answer of "Undyne", showing she bears some attraction towards Asgore as well. She has a hidden side of herself that is filled with guilt and insecurities about herself and her accomplishments. In particular, she is unintentionally responsible for the miserable existences of Flowey and the Amalgamates. The majority of this side of her is revealed in the True Laboratory.
Frisk
Frisk talks to other characters, but unless the player receives a prompt on what to say, their dialogue is not shown. During battles, the flavor text describes what Frisk says. Outside of battle, when Frisk speaks on their own, the other character's dialogue box displays "..." to indicate them listening to Frisk. Depending on the ending that the player is moving towards, Frisk can be depicted as friendly, in the case of the Neutral (or especially the Pacifist Route). In the Pacifist Ending, characters such as Asriel make emphasized note about Frisk's kindness, understanding, and grace. This is also the only ending when Frisk's name is given, which implies that this is their true character and personality. In the case that the player chooses the Genocide Route, Frisk comes off as cold, eerie, and sinister to all that meet them. Towns are evacuated and monsters are recommended to not interact with Frisk in any way possible. Flowey and Sans perceive the character as not being human (in the sense that they lack any humanity) and Asgore does not recognize them as human (in other playthroughs, he immediately recognizes them). It is, however, unclear how much of Frisk even remains at this point: many interactions with the environment, starting from Toriel's house, suggest that on this route Frisk is, in fact possessed by the first human. Loading saves or resetting will not affect Frisk's memory of certain events. Some characters notice Frisk behaving strangely as a result of this. It should be noted Frisk is designed for players to put themselves in the shoes of Frisk. Therefore, their personality is not shown very well in the game by canon alone, and is truly up to the interpretation of the player. Abilities Edit