Fanart involving the Player sometimes depicts them as using the grey discarded Vessel from the start of the game. The SOUL also carries an association with red "strings" connecting to Kris and other characters, thanks to all the puppet motifs that Spamton NEO's fight provides.
While in-game their relationship is kept vague, fanworks generally agree that King and Queen are ex-lovers and Lancer's parents.
The Three Heroes
Kris:
Kris' Dark World appearance can be depicted with dark blue sleeves which would match their bodysuit despite the arms' sprite being light blue, indicating that they don't wear sleeves at allnote The dark blue bodysuit may be inspired from Kris' back sprite, where their cape covers their arms and makes them darker as a result.
While official art and merchandise depicts Kris' Dark World cape as a solid reddish-pink, a good chunk of fan artists depict it as having pink and purple stripes in the vein of Frisk's shirt in Undertale, based on the purple shading it has on Kris' in-game sprites (and to tie it in with their striped shirt in the Light World and their parallels with both Frisk and Chara). It helps that Toby Fox has made the canonicity of the merch for his games rather dubious. Similarly, Kris's Light World appearance is often portrayed in fanart with a black hoodie or dress shirt underneath their sweater, sneakers, black nail polish and/or black fingerless gloves, due to the simplicity of their design giving artists opportunities for interpretation.
Their gender:
In contrast to Frisk, who is occasionally depicted in the fandom as female when not portrayed as gender neutral, Kris is often depicted as male when they're not referred in gender-neutral pronouns. While this may be due to their knight outfit in the dark world (which is considered more masculine than, let's say, Ralsei's wizard dress), the name "Kris" being seen as a masculine name, or the fact they share a room with their brother Asriel, this interpretation has been enforced since players discovered the Weird Route where they abuse Noelle in surreal and disturbingly realistic ways, making the audience think about the most common depiction of abusers.
Female interpretations still exist though, especially for those who think of Kris as an alternate universe teenage Charanote a character who's commonly depicted as female or for those who ship Kris and Susie.
Susie:
Linked to the previous point, Susie is often interpreted as a Butch Lesbian, despite her canon self only being Oblivious to Love.
Before the release of Chapter 2, fans also tend to debate on the character having a tail or not, as her butt is always covered by a jacket.
While it's unclear what kind of a monster she exactly is, most fanworks interpret her as a lizard- or dinosaur-like creature.
Ralsei:
Despite their in-game sprites being around the same size (if you don't count the horns), a lot of fanart depicts Ralsei as shorter than Kris, to make him look cuter and to make the Three Heroes a Big, Thin, Short Trio (with Kris as thin and Susie as big). This seems to be a remain of Ralsei's hooded design where he looked shorter without his hat◊; and this picture where he looks the shortest because of the hood.
Since Chapter 2, people tend to argue on Ralsei's dress black motif which can either be seen as a heart or a spade turned upside-down.
Lightners and Light World
If any of the original Undertale cast ever entered the Dark World, it's generally assumed that they would gain costumes akin to what they wore in the original game, such as Toriel and Asgore sporting their royal garbs.
Noelle's family:
Although she hasn't made any physical appearance, most fanart tends to give Dess black hair in order to match with her father, Rudy. She's also often portrayed as tomboyish to constrast with how girly Noelle is.
Less commonly, Noelle's mother tends to get blond hair in order to match Noelle's.
Many fanart depictions of this world's Asriel are given glasses to strengthen the parallels with Ralsei.
Rouxls Kaard is often depicted as being made of inky goo. This likely stems from him looking very human; fans have used the bit of "skin" connecting his lips, which can be interpreted as a strand of slime, as a reason to see him as a more nonhuman creature.
Seam:
Seam frequently gets portrayed as having an Irish accent, thanks to their name being a play on the Irish name "Seán."
It's a common concept for Seam to initially have had two button eyes, their missing one being caused by an attack from Jevil after he went insane.
The idea that Jevil has devil horns under his jester hat is generally accepted. Less often, he's depicted as having hair too.
It's common to portray the King of Diamonds as the father of the Rudinns, the King of Hearts as the father of the Hathys, and the King of Clubs as Clover's father.
Chapter 2 Darkners and Dark World
Spamton:
As Spamton was friends with the Addisons and shares parts of their design, fans try to depict what his form as an Addison must have looked like before he began poorly copying Swatch. He's generally depicted as a short Addison with white skin and hair, and lacking his rosy cheeks and glasses. A poster near his dumpster shows that his past self simply looked like a jollier version of his regular self without the Dealmaker glasses, though that doesn't stop anyone. To stay in line with this, some fans also depict him as having adopted his poster appearance once he hit it big.
Since all the other Addisons have hair the same color as their skin, it's a very common idea in fanworks that Spamton has naturally white hair, but either dyed it black or was transformed by his benefactor's influence.
Some fans also like to imagine the Addisons and Spamton as siblings, adding an extra layer of sadness to them essentially abandoning him during his backstory.
Thanks to Spamton's ventriloquist dummy-inspired design, fan artists frequently depict him as being around as small as an actual dummy, despite being close to Kris' height in-game. He's often also drawn with doll-like joints in his limbs.
While depicted as a case of Painting the Medium for his Electronic Speech Impediment in-game and never fully clarified beyond that, fans like to portray the bracketed segments of Spamton's dialogue as being spliced in from various advertisements and programs (often ones that audibly differ from his "regular" voice), tying in with his glitchy, spambot-inspired nature and personality as a salesman.
Fans believe that the reason why Spamton is so short compared to the Addisons is that he was dunked in Queen's acid pool, coming from Spamton's shop dialogue and Queen's claim that the acid pool can shrink people. He's sometimes portrayed as the same size as the Addisons before his Start of Darkness. His Puppet Permutation is also sometimes explained as his skin being burned off by the acid, revealing a puppet-like skeleton. Spamton Sweepstakes would later seemingly joss the acid theory, revealing he was always short, but that he did likely physically change in some other way during his time as a big shot.
When Spamton is shown without his glasses in fanworks, he often has heterochromia and his eyes are pink and yellow like his glasses.
Fanworks often give Spamton severe Height Angst, regardless of whether they subscribe to the acid theory or have him always be short, thanks to how much taller he is as Spamton NEO.
A musical example: covers of "BIG SHOT" often end with the final mesures from "Dummy!" despite the original song only taking the mesures before these.
The implications of Queen's speech pattern of Capitalizing Every Word And Using Barely Any Punctuation haven't been fully clarified, and as such fans interpret it in multiple ways. One way is to portray her with a Robo SpeakMachine Monotone, which also fits with how her dialogue is translated as Faux Computer Code in the Japanese translation. Another way runs on Rule of Funny and literally interprets her capitalization patterns to Over-Enunciate Every Word As Its Own Sentence, another form of Robo Speak that makes her sound vaguely like a cartoonish William Shatner impersonation.
While Swatch and the Swatchlings appear identically-sized in their overworld sprites (which can be directly compared in Castle Town's café), fans frequently portray Swatch as shorter than the Swatchlings to further the comparisons to Spamton, who's noticeably shorter than the Addisons.
The Addisons are often given birdlike features such as feathers in fanart, due to their pointy noses resembling bird beaks and due to their association with Spamton, who himself carries a large number of avian motifs. Sometimes, the Pipis are portrayed as actual eggs that they lay.
When the Addisons are given different personalities in fanworks, the pink Addison is typically portrayed as flirty and campy, due to associations with their pink coloration, long hair, and speech style, such as calling Noelle "Angel", the blue Addison is typically calm and the nicest one, due to them being the one who went to check on Spamton for the last time, as well as the color blue being associated with calmness, and the yellow Addison is often portrayed as extroverted and assertive, due to them being yellow and having Spiky Hair. The orange Addison doesn't seem to have a concrete fanon personality.
While Tasque Manager has Floating Limbs, fan works depict her with only floating hands while leaving the rest of the body unified.
While not exactly universal, many fanworks featuring the Addisons give them short online media-related names. Specifically, the names Survey, Click, Audio, Video, Popup, and Banner are used in a lot of fanworks.
of the merch for his games rather dubious. Similarly, Kris's Light World appearance is often portrayed in fanart with a black hoodie or dress shirt underneath their sweater, sneakers, black nail polish and/or black fingerless gloves, due to the simplicity of their design giving artists opportunities for interpretation.
