Undertale looms large in the hearts of its fans for many reasons, but arguably due mostly to the strength of its characters. These beloved monsters each inspire their own devoted fan communities, with perfect cosplay and fan art to be had in abundance.
Not only are each of the characters well fleshed out — even the skeletons — but they also vary in the extreme. Their personalities run the spectrum of the Myers-Briggs Type Indicator®, although some types may dominate the aesthetic more than others. Those who have yet to play through this beloved tale be warned — from here on there will be spoilers!
16 Alphys - INTP
Alphys is the poster child for introverts, and her great intellect as a Royal Scientist makes her an INTP.
INTPs are excellent analytical puzzle solvers but can fail to consider the emotional fallout of a solution, somewhat like Alphys and her work related to monsters and Determination. INTPs can be extremely shy and withdrawn but can be very excited when they are interested in something. Alphys is something of a shut-in but she will happily babble at length about Mew Mew Kissy Cutie.
15 W.D. Gaster - INTJ
Gaster was the mysterious former Royal Scientist who created the Core and may have been investigating Determination's effect on monsters before Alphys took over. Based on his legacy, Gaster is a brilliant scientist who rose to the top of his field. He casts a singular presence over the game, which is why he is an INTJ.
Little is known about Gaster, as he is mostly absent from the game itself, similar to INTJs tendencies to withdraw from the world.
14 Napstablook - ISFP
Artistic and individualistic to the max, the ISFP types are embodied in the character of Napstablook. This loveable ghosty is a music producer and lover and completely unconventional — at one point, the player can choose to lie on the floor with Blooky, and dissociate to trance music together.
ISFPs are vulnerable to bouts of low self-esteem; Napstablook cries during fights and does their best to be unassuming throughout the game.
13 Mettaton - ESFP
Mettaton isn't shy about sharing tidbits about himself in the game. He is the most obvious ESFP character — but then again being obvious is kind of an ESFP type's thing.
Novelty is a big draw for ESFPs, and, of all the monsters the player has to face multiple times, Mettaton offers the most variety in his battles. The player fights Mettaton through walls of fire, a romantic Broadway show number, and a taping of a cooking show, to name a few of the most memorable battles.
12 Sans - ENFJ
ENFJs are charmers with a tendency to take responsibility for everyone else's emotional state; Sans is everywhere in the game, watching over Papyrus and the other monsters, and he oozes a particular kind of pun-filled charm.
ENFJs are very open-minded although this can lead to them being too idealistic. Depending on how the player chooses to act in battle, this quality can be a boon or a mistake when Sans is dealing with Frisk.
11 Bratty And Catty - ENFP
Happy-go-lucky and upbeat, ENFPs are able to put a smile on anyone's face, especially their own, and these two monsters have a rapport that is entirely delightful.
ENFPs are also known for inventive problem solving; creating a business with goods scavenged from the garbage dump proves B&C are both ENFPs.
10 Toriel - INFJ
Toriel is a perfect representation of an INFJ. Altruistic INFJs are natural teachers, and Toriel is the one who originally teaches the player how to play as a pacifist.
INFJs adhere strongly to their beliefs and are passionately idealistic even in the darkest of circumstances. Toriel refuses to aid her husband and king's cruel plan, even in the depths of her despair over Asriel.
9 River Person - ISTJ
The River Person has only one role in the game, shepherding the player back and forth between the various sections of the game (and singing). Their devotion to iteration and consistency makes them an ideal ISTJ.
ISTJs are known for their devotion to duty, but they can be hard to get to know. It takes the player many trips down the river to learn much about the River Person, but they are always there waiting patiently when needed.
8 Undyne - ISFJ
Undyne the Undying is an ISFJ — one of the best types to have your back. She's willing to defend her loved ones to the death. In fact, she's willing to defend all the monsters to her death. This tendency towards overdoing altruism at the expense of herself is a classic ISFJ move.
ISFJs can have slightly contradictory qualities depending on the situation. The player can experience both sides to Undyne in Waterfall, by both flirting with her and fighting her.
7 Asgore - ESFJ
As the leader of the monsters, Asgore is respected, but mostly he is beloved. Most of the monsters make a point of saying what a nice guy he is.
ESFJs have a desire to do right by everyone that can often put them in a difficult position when faced with tough decisions. Asgore's choice to do his duty to use human SOULS to help the monsters leads him down a dark path.
6 Asriel - INFP
Asriel doesn't appear in person through much of Undertale, although there are plenty of hidden details about him to pick up on throughout the game. From the few scenes he has, he is clearly an INFP. A sweet, simple child, Asriel is devoted to his family and full of love.
But these qualities also make him emotionally vulnerable and open to be manipulated. Asriel's altruistic desire to please his beloved adopted human sibling leads to his own destruction.
5 Chara - ENTJ
ENTJs are natural leaders, driven and passionate. Chara doesn't feature much in the game, but they are the driving force behind the narrative. What the player does learn about Chara indicates that they were excellent at bending others, such as Asriel, to their will.
ENTJs can get a reputation as somewhat ruthless in pursuit of their goals, and Chara is no exception. Although their motives may have been to save all the monsters from human subjugation, Chara's resulting hatred of humanity leads everyone down a dark path.
4 Flowey - ENTP
Flowey is a diabolical philosopher; few video game villains will make a player question themselves and their motivations like Flowey. For this reason, he is a clear ENTP, a type that loves a good argument, particularly because they rarely lose.
As a damaged soul, Flowey trends towards the worst qualities of an ENTP: he is intolerant, cruel, and unconcerned with the existence as a practical matter. Should the player indulge in similar awful behaviors, Flowey will prove to be a merciless mirror.
3 Muffet - ESTJ
Muffet is a strong, boss-like character who won't shy away from using brute force when bending the player to her will. She's a leader and uniter of the spider community in the underground.
Similarly, ESTJs are excellent organizers and capable leaders but can be quite unscrupulous in their methods of achieving success. Muffet switches alliances immediately once she finds out Frisk can be useful alive.
2 Papyrus - ESTP
Papyrus is rowdy, energetic, and original, making him an ESTP. Although they can sometimes be insensitive and blunt, ESTPs place great importance on connections with others. Papyrus will obtusely twist any insult into a compliment, but with purely endearing results for Frisk and the player.
ESTPs will take the most direct route to any goal, and this can sometimes lead to risky behavior, just as Papyrus pushed to have himself made a member of the Royal Guard, despite his extreme lack of human-spotting abilities.
1 Annoying Dog - ISTP
The Annoying Dog is somehow elusive and also everywhere at once — or at least his residue is. He is not overly concerned with the player's adventures, but his presence is notable, whether he is functioning as a bomb, or stealing bones from battles. But it is AD's status as creator Toby Fox's avatar that cements his role as an ISTP.
Reclusive and highly skilled, ISTPs are creative and inventive, perfect qualities to be a solo game dev. They are often a lone wolf to the max — or lone dog.