In 2026, I still keep coming back to Undertale for the same reason I did on my first playthrough: the monsters feel impossibly alive. More than a decade after its release, the game’s cast remains a favorite subject for fan art, cosplay, and endless personality debates. In this first-person tour, I am looking at the beloved monsters through the Myers-Briggs Type Indicator lens. This is fan interpretation, not canon, and it comes with spoilers for the full game.

I find the MBTI exercise useful because Undertale’s characters are not just well fleshed out; they also vary in the extreme. Their personalities run across the spectrum, even if some types dominate the game’s aesthetic. When I replay the game in 2026, I see a cast that still rewards close reading. The typings below are my own fan reading, built from the characters’ choices, humor, emotional labor, and the small details that make each monster memorable.

Character	Type	Why I see it
Alphys	INTP	Analytical Royal Scientist, shy shut-in, excited babbler about Mew Mew Kissy Cutie
W.D. Gaster	INTJ	Brilliant former Royal Scientist, singular presence, mostly absent from the world
Napstablook	ISFP	Artistic ghost, music lover, vulnerable self-esteem, deeply unconventional
Mettaton	ESFP	Showy, novelty-seeking, variety-driven performer
Sans	ENFJ	Pun-filled charmer, watches over others, open-minded but idealistic
Bratty and Catty	ENFP	Upbeat pair, delightful rapport, inventive scavenger business
Toriel	INFJ	Altruistic teacher, idealistic, refuses to aid a cruel plan
River Person	ISTJ	Dutiful ferry guide, consistent, patient, hard to know
Undyne	ISFJ	Loyal defender, self-sacrificing, contradictory in different situations
Asgore	ESFJ	Beloved leader, wants to do right by everyone, burdened by duty

The shy analysts

Alphys is my poster child for introverts, and her great intellect as a Royal Scientist makes her an INTP. I see the type clearly in her analytical puzzle-solving mind, but also in the way she can fail to consider the emotional fallout of a solution. Her work related to monsters and Determination carries that tension. INTPs can be extremely shy and withdrawn, yet very excited when something captures their interest. Alphys is something of a shut-in, but she will happily babble at length about Mew Mew Kissy Cutie. That contrast is exactly why she feels so human to me.

W.D. Gaster is 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, I read Gaster as a brilliant scientist who rose to the top of his field. He casts a singular presence over the game, which is why I see him as an INTJ. Little is known about Gaster because he is mostly absent from the game itself, similar to INTJ tendencies to withdraw from the world. In 2026, he remains one of the fandom’s favorite enigmas.

The artistic outsiders

Napstablook is artistic and individualistic to the max, which is why the ISFP type fits so well for me. This loveable ghost 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. I always leave that encounter feeling oddly protective of them.

Mettaton is not shy about sharing tidbits about himself, and he is the most obvious ESFP character. Being obvious is kind of an ESFP thing, after all. 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. I fight him 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. His hunger for spectacle makes him a perfect showbiz ESFP.

The social charmers

Sans is an ENFJ in my reading. 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. I find that tension central to his appeal.

Bratty and Catty are happy-go-lucky and upbeat, and ENFPs are able to put a smile on anyone’s face, especially their own. 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 to me that Bratty and Catty are both ENFPs. Their shop is silly, but it is also a small lesson in making something out of nothing.

The caretakers and the duty-bound

Toriel is a perfect representation of an INFJ for me. 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. That quiet refusal is one of the most powerful moments in the game.

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.

Undyne the Undying is an ISFJ, one of the best types to have your back. She is willing to defend her loved ones to the death. In fact, she is willing to defend all the monsters to her death. This tendency toward 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 of Undyne in Waterfall by both flirting with her and fighting her.

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. I read him as a tragic ESFJ: warm, responsible, and caught in a role that asks too much of him.

In 2026, I do not treat MBTI as a science, but as a playful critical lens. Undertale’s writing remains strong enough that fans can still argue about these typings with real evidence from the game. That is why I keep returning to these monsters: their personalities are broad, contradictory, funny, and sad in ways that make personality analysis feel less like a label and more like a conversation. 😄