Toriel and Asgore Dreemurr

The Queen and King of the Underground in Undertale, they are members of a powerful, goat-like species called Boss Monsters. Asgore rules over the monster city of New Home. Toriel split from Asgore due to his desire to kill humans, collect their souls, and free the monsters by force following the deaths of their children Asriel and Chara, and now lives alone in the Ruins.

Toriel initially attempts to adopt the protagonist as her child. When the protagonist tries to escape, she acts as the game's first major boss, attempting to prevent the player from entering the Underground and falling prey to Asgore. It is not made immediately obvious she can be spared, making her accidental death a common occurrence. The tendency of players to reload in an attempt to save her is one of the game's first demonstrations of Flowey's "meta" knowledge, as he will remember if the player has ever killed Toriel in the past. In the game's Toriel Ending, Toriel reclaims her position as Queen of the Underground and makes a rule where all humans will be treated as friends. This ending can play out in two different ways, with her either remaining in power or abdicating following a revolution (spearheaded by Undyne if she was spared) and returning to the Ruins (where she will be accompanied by Sans, and additionally Papyrus if he is spared).

The character of Toriel has received attention from critics for her personality and atypical moral choice of her boss battle. Nathan Grayson of Kotaku stated that while he killed Toriel during his playthrough, his encounter with her made him cry due to her friendliness and relatability as a character.^[1] He also praised the game for remembering this on his next playthrough.^[2] Julie Muncy of Kill Screen criticized the encounter with Toriel as unintuitive, saying that she lost the ability to trust the game after seemingly having to kill her, which forced her onto the game's "Neutral" route.^[3] Jess Joho from the same magazine called the character "relying on the inhumanly selfless portrait of motherhood", but doing so with a purpose, praising the boss encounter with her as respecting the player's ability to think through a problem, as well as their basic instincts as a human being. She stated that the game holds up a mirror to "patriarchal" game design that encourages players to impatiently "sacrifice their own mother and humanity" rather than engaging with the enemy on a "human level".^[4]

Asgore is encountered late into the game, and serves as the penultimate boss. Despite a fearsome reputation in battle, he is a kindly father figure in person, and is heavily reluctant to go through with his plan of destroying humanity, feeling regret for having killed the six humans that arrived before the player. He is finished off by Flowey if the player attempts to show him mercy, but survives in the game's True Pacifist Route after Toriel's intervention.

In Deltarune, Toriel and Asgore are Kris' foster parents. Toriel is a teacher at a local school. Asgore remains divorced from Toriel, and runs a flower shop known as 'Flower King', referencing his Undertale incarnation's gardening hobby and being the King of the Underground. Asgore was previously the chief of police before being fired following an unknown incident.

Asriel Dreemurr

In Undertale, Asriel is the now-deceased prince of the Underground who was convinced by Chara, his adoptive human sibling, to participate in a plot to collect six human souls from the surface. Upon absorbing Chara's soul and exiting the Underground, Asriel refused to fight back against the humans who mistook him for Chara's killer, killing them both. Asriel was then reincarnated as Flowey, but bereft of all empathy and compassion. In the game's True Pacifist Route, Flowey turns into an adult representation of Asriel, the "God of Hyperdeath" or ''Angel of Death''. When defeated, Flowey returns to the form of child Asriel and regains his empathy.

Jason Schreier of Kotaku called Asriel's final boss fight one of the greatest in video game history, citing its climactic music and the necessity to save your friends from Asriel. Describing the sequence as "spectacular" and noting the emotion of the fight, he stated that, "if you’re not already bawling as you mash the SAVE button over and over again during the final part of this fight, well... you will".^[5]

In Deltarune, Asriel is Kris' brother, but has left for university and is not seen in person.

Papyrus and Sans

Two skeleton brothers known for their outlandish human-capturing schemes, they function as comic relief within Undertale. Their names each correspond to a widely maligned typeface, Papyrus and Comic Sans, which they use for their dialog. Papyrus is egotistical yet incompetent, has a strong love for puzzles and pasta, and desperately wishes to join the Royal Guard; despite this, he is shown to be a kindhearted and genial person who believes that anyone can be good if they try. Sans, meanwhile, is a clever, laid-back slacker and trickster who often pulls practical jokes, and appears to sometimes break the fourth wall. Sans bonded with Toriel over their love of jokes.

In Undertale's Genocide route, Sans functions as the game's final boss, revealing he is secretly an incredibly skilled fighter with control over the game's combat system, even breaking how battles work. While statistically weak, he uses a unique "Karma" mechanic to deal damage per frame and pure speed to dodge the player's attacks. The fight's theme music, "Megalovania", became a widely renowned Internet meme. In the Papyrus Ending, Papyrus becomes leader of the Underground with Sans doing the work, but reveals that he is nervous about the job due to being unaware all the other main characters had been killed by the player.

In Deltarune, Sans runs a grocery store simply known as 'Sans, which outwardly resembles "Grillby's" from Undertale with the "Grillby" crossed out. When talking to Kris and Susie in Chapter 2, he refers to himself as its "janitor", although his wording and dialogue during this part is deliberately confusing by him. Papyrus is not seen, but implied to exist as Sans mentions having a brother, alongside some dialogue referring to Papyrus' main battle theme "Bonetrousle".

Undyne

In Undertale, Undyne is the immensely powerful leader of Asgore's Royal Guard and leader of the Underground (Undyne Ending or alternate Toriel Ending). She is largely humanoid, but with fish-like features. While initially presented as an intimidating figure, her ham-fisted behavior in combat is heavily inspired by anime, which she falsely believes to be an accurate representation of the human world. Undyne possesses a small amount of Determination, a substance that makes humans far stronger than monsters, that enables her to persist upon death. It is theorized this is due to the intervention of her love interest, Alphys, who was conducting experiments on injecting Determination into vessels such as monsters and flowers (which is how Flowey was created).^[6] Compared to the remorseful Asgore, Undyne has a deep hatred of humans and will not hesitate to kill them.

In Deltarune, she is the town's police chief after Asgore was fired.

Alphys

In Undertale, Alphys is Asgore's Royal Scientist as well as the love interest of Undyne. She is a shy, nerdy, lizard-like monster with low self-esteem, and a heavy otaku. She created Mettaton's robot body at the cost of abandoning his cousin Napstablook. She tries to earn the protagonist's trust by helping them "fight" off a malfunctioning Mettaton throughout Hotland, but he later reveals he was simply acting to make Alphys look better. She is the successor of W. D. Gaster, who served as Royal Scientist until he mysteriously disappeared.

It is revealed in the True Pacifist route that she experimented with Determination on comatose monsters, but it ended with them melting altogether and fusing into creatures known as the "Amalgamates". She keeps them locked away in the area called the "True Lab", which can only be found on a True Pacifist route where she reveals her secrets and resolves to improve her self-esteem. She becomes the leader of the Underground in the Alphys Ending, and is implied to have committed suicide in any ending where Undyne and/or Mettaton die.

In Deltarune, she is another teacher at Kris' school, and is quite nervous around Susie.

Mettaton

In Undertale, Mettaton is a robotic entertainer who is the Underground's sole celebrity, various television shows, products and brands, and a hotel in Hotland. An egotistical glory-seeker, he wants to take a human soul to travel to the surface and become a celebrity amongst the humans, and tricks Alphys into helping the protagonist to reach the CORE so he can kill them under the pretenses of making her look like a hero defeating him. It is revealed through diaries in an abandoned house that Mettaton was originally a ghost who dreamed of creating a perfect body for themselves, and got their chance when they befriended Alphys at the cost of abandoning their cousin Napstablook. In the Genocide Route, Mettaton transforms into a seemingly powerful form called Mettaton NEO, but is killed by the player in one hit. In the True Pacifist Route, he is reunited with Napstablook, who becomes his sound mixer. In the Mettaton Ending, Mettaton becomes the Underground's dictator, building a cult of personality around himself and making those who do not worship him "disappear", with Sans and optionally Papyrus serving as his bouncers.

In Deltarune, while Mettaton is not seen, they are almost certainly the ghost behind the door of Napstablook's house.

Napstablook

In Undertale, Napstablook is Mettaton's cousin, an introverted ghost and DJ. The protagonist battles them in the first area of the game, and can befriend them later at their house in the third area.

In Deltarune, they work as a police officer with Undyne, but do not notice that all of the town's criminals have escaped until the protagonist talks to them.

Mad Dummy/Mad Mew Mew

Mad Dummy is a character located in Waterfall presumed to Napstablook's cousin and therefore Mettaton's cousin. They are a ghost that is attempting to become corporeal with their dummy body (succeeds in the Genocide Route). They are battled before reaching the more populated area of Waterfall.

In the Nintendo Switch and Xbox ports for the game, the ghost steals a doll of the titular character of the anime "Mew Mew Kissy Cutie" from Alphys's lab en lieu of the dummy and takes on the name Mad Mew Mew, believing it to represent their true form better. In the Nintendo Switch port, Mad Mew Mew fights the player character to try and merge with the doll in the Pacifist run if they investigate under Papyrus's sink, engaging in a unique soul mode utilizing the Nintendo Switch's Joy-Con controllers (and will appear outside Undyne's house in the Pacifist ending regardless of if you battle her). In the following ports, she has already merged with the doll without the player's interference.

In Deltarune, they are not seen, but are very likely a ghost also found in the same house as Mettaton in Chapter 2.

Frisk

A child from the Surface and the main playable character of Undertale, they are initially presented as a player-nameable character also known as the "fallen human". In a late-game plot twist exclusive to the True Pacifist Route, it is revealed that the "fallen human" only superficially resembled Frisk, but had arrived years earlier. Frisk has an androgynous appearance, and is a silent protagonist, leaving their gender and personality for the player to decide. They spend the game searching for a means to escape the Underground.

Chara

A child from the human world taken in by Toriel and Asgore. They are nameable by the player, but Chara is referred to as their "true name" on the naming screen. It is stated that they climbed Mount Ebott for an unhappy reason and that they hated humanity. Chara enacted a plan to collect six human souls by committing suicide, letting their foster brother, Asriel, absorb their soul and exit the Underground as one being. The humans fatally wounded them, leading to their deaths and Chara's burial in the Ruins. In the game's Genocide route, Chara is resurrected as a soulless and demonic being who seeks to erase the world, referring to themselves as "the demon that comes when people call its name". There have been theories in the Deltarune community that Chara is in Deltarune, but they have yet to make an appearance.

Flowey

Flowey is a flower who initially pretends to be kind-hearted, but strictly adheres to a "kill or be killed" philosophy and once maintained the ability to operate outside the laws of the universe via saving and loading save files much like the player character is able to. A powerful, eldritch version of him known as "Photoshop Flowey" or "Omega Flowey" serves as the final boss of the game's Neutral route. If he is spared after his fight as Photoshop Flowey in the neutral route, he says he cannot understand the player character's kindness towards him in spite of his threat to murder everyone they love. Flowey is later revealed to be the reincarnation of Asriel Dreemurr, the deceased prince of the Underground. Flowey, like Undyne, possesses Determination (although it is not seen), as Alphys injected it into one of Asgore's first golden flowers during her experiments.

W. D. Gaster

W. D. Gaster is a mysterious character noted for being the former Royal Scientist who created the CORE (the Underground's power system), and for his abrupt and mysterious disappearance, after which he was replaced in his post by Alphys. Gaster's presence in the game is minimal, only being directly mentioned in some rare Easter eggs and internal game data discoverable through datamining.

Events and encounters relating to Gaster are restricted to a system within the game known as the "Fun value", which randomly selects a number at the start of a playthrough, triggering small events along the way depending on which number was picked. If an appropriate Fun value is assigned, the player may encounter characters that give backstory relating to Gaster, establishing his presence as a character in the story of the game. These are known as "Gaster followers", telling the player that Gaster "shattered across time and space", that "his experiments went wrong", or that he "fell into his creation". One of the followers implies that Gaster still lives, noting that "it's rude to talk about someone who's listening."

Through manual editing of save data, players can also access an otherwise hidden room called room_gaster. This room displays text written in the Wingdings font, titled "Entry number 17", in which an unknown character (presumably Gaster) monologues about experiments involving darkness. Though Gaster has no clear canonical appearance, fans generally associate him with a warped sprite known as "Mystery Man" which has a chance of appearing as one of the Fun value events. Other elements of the game also hint at Gaster's involvement, such as the weapons used by Sans at the end of the Genocide route being known internally as "Gaster Blasters", or the fact that if a player opts to name the fallen child Gaster, the game will forcibly restart itself, preventing the name from being used, although when a save file is edited to have the name Gaster, resetting is no longer possible as doing so sends the game back to the opening introduction cutscene.

Despite his near-complete absence from the game, the vague yet striking presence of Gaster in Undertale has created a sizeable fan following around the character, turning him into something of an urban legend. Certain clues hint at Gaster also being involved in the events of Deltarune, but his presence has yet to be explicitly confirmed, although the original website for Deltarune does feature a part of entry number 17.^[7][8]

Kris

An androgynous, introverted teenage human who was adopted by the Dreemurr family and the main character of the game. They are stated to have a trickster personality, and are generally perceived by others as a somewhat troubled youth.

While Kris is the player character, it is important to note that there exists a separation between Kris as a character and the player as a separate entity. Kris is normally controlled through a red soul, making use of the player's inputs, though the player's control can jump to other characters if necessary. Other characters often note the absence of Kris's usual personality during the player's influence, and there are multiple hints shown throughout the two chapters that Kris harbors their own thoughts and feelings outside of the player, and even outright contradict some of the player's choices.

Kris is able to remove their soul from their body and exercise free will outside of the player's control, implying it is not their own soul. During the ending sequence of Chapter 1, Kris throws the soul into a birdcage, at which point the player's movement inputs control the trapped soul, not Kris themself. At the end of Chapter 2, Kris is shown to create a Dark Fountain when not controlled by the player.

In the secret "Weird" route, the player manipulates Kris into forcing Noelle to fatally freeze Berdly and many Darkners. In the secret Spamton NEO fight (which also becomes the chapter's final boss when the player is on the "Weird" route), the soul can turn yellow and fire shots akin to the yellow soul in Mettaton EX's boss fight in Undertale.

Lancer

Lancer is the miniboss of Chapter 1 who befriends the protagonist and becomes a recurring character in Chapter 2. He is the son of King, the final boss of Chapter 1. He is energetic yet incompetent in his attempts to stop the heroes to earn his father's respect. His gradual developing friendship with Susie is eventually what causes him to later rebel against his father.

King

King is the final boss of Chapter 1, and is the father of Lancer. He is shown to be a tyrannical figure with little obvious care for his subjects or family, is zealously dedicated to the Knight, and plans a campaign to take over the Light World as retribution for his abandonment by the Lightners. He has a rotund body and a second mouth on his stomach with a spade sticking out of it. He is also sometimes known as ''Chaos King'' or "King of Spades".

Queen

Queen is the main antagonist and final boss of Chapter 2. She is a robot who attempts to capture the party multiple times, although she has no ill will towards them and is mostly a friendly if eccentric figure. Her full name is "Serial Number Q5U4EX7YY2E9N" and she possesses a gigantic robot called GIGA Queen. She is Lancer's adoptive mother, although whether she is his biological mother is unclear.

Ralsei

The prince of the Dark World, he wears a green robe and wizard-like hat that covers him entirely in shadow in Chapter 1. He tells Kris of the threat posed by the Dark Fountains, and appears to be meek and kind-hearted, encouraging the player to deal with enemies in a non-violent way. When the hat is removed, he is revealed to look similar to Asriel, in addition to their names being anagrams.

Susie

A purple, alligator-like fellow student with great strength, and a gruff, tomboyish appearance and demeanor. She uses an axe, and becomes one of the player's main party members. She starts out aggressive and unwilling to use mercy or befriend people, but mellows out over the course of Chapter 1 and becomes more friendly if boisterous in Chapter 2. She befriends Lancer and Kris, becoming particularly close with the latter, over the course of the game's Chapter 1, while she tends to be playfully snarky towards Ralsei.

Noelle Holiday

A timid fellow student resembling an anthropomorphic reindeer, she joins the player's party in Chapter 2 and is a childhood friend and classmate of Kris. In the Dark World, she wears a white robe and has proficiency in magic. The player is able to use her "IceShock" spell to freeze enemies solid, and, if a certain sequence of events is followed, prompt a darker secret route in which Berdly is trapped in ice with the "SnowGrave" spell and she is forced to become a subservient follower for the player. She also has a crush on Susie. She has an older sister nicknamed "Dess", implied to be short for December, who is often mentioned but never seen.

Berdly

A know-it-all student resembling an anthropomorphic bluebird who becomes Kris' rival in Chapter 2, and later their ally. It is revealed that Noelle is actually smarter than him, but he has continued his snobbish persona in order to gain attention and praise. If frozen solid in the Weird route, he does not reawaken in the Light World.

Seam

The first shopkeeper encountered in the game, Seam is a nihilistic, non-binary Darkner who is a personified cat plushie with a missing eye. If the player pursues Jevil's sidequest, it's revealed they are the former Court Magician who quit after having to lock up their friend. In Chapter 2, Seam continues to be a shopkeeper in Castle Town and reveals that all superbosses possess powerful artifacts known as "Shadow Crystals", claiming they could make "something incredible" if the player collects them all.

Jevil

A Darkner who is a personified Joker card optionally encountered in Chapter 1. The former Court Jester and partner of Seam, he was locked away after an interaction with a strange unknown figure, implied to be W.D. Gaster, drove him to madness. He began to see his whole world as merely a game and all its inhabitants merely parts of the game, thus influencing Seam to adopt a similarly nihilistic outlook on life. Despite being locked in a cell, he now believes all the other inhabitants of the Dark World have imprisoned themselves and he is the only free Darkner remaining. Upon being set free, he challenges the party to a battle as a "game" and serves as the superboss of Chapter 1.

Spamton G. Spamton

A personified spambot Darkner with a marionette-like appearance encountered in Chapter 2, he speaks in mangled, cryptic English peppered with bracketed marketing keywords in the manner of a spam email. Spamton has the personality of a used car salesman, and is obsessed with becoming a "BIG SHOT". He often states that he and Kris are alike in some way and that he is trapped as well. He plays a role in unlocking the chapter's hidden route by selling the player the Thorn Ring, and also functions as that chapter's superboss, Spamton NEO, after merging with a defunct robotic body resembling Mettaton NEO; he is also capable of accessing this form in the normal route by performing certain actions in his shop and in Queen's Mansion. Upon defeating Spamton NEO in the normal route, the player can revisit his shop and speak with his former coworkers, who explain how he rose to success after communicating with an anonymous voice on the phone who guided him to success, only to lose his job, home, and sanity when the figure abandoned him. It's hinted that Spamton met the same figure that drove Jevil mad.

The Roaring Knight

The Roaring Knight is the overarching antagonist of Deltarune. A mysterious figure, they are the one who creates the Dark Fountains and the Dark Worlds they encompass. They appear to be doing this to bring about the "Roaring", a prophesized calamity caused by excessive Dark Fountains creating an imbalance and drowning the world in darkness. King and Queen show great deference to the Knight, regarding them as a figure to worship and emulate. It is implied the Knight possesses high-enough Determination to create Fountains. Their identity is unknown.