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What we know for sure:
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─ sans instantly recognizes the Player as a human and has knowledge of the Surface.

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─ In the conversation at MTT Restaurant, sans confesses that he knows the feeling of wanting to go home.

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─ According to the Snowdin Shopkeeper, sans and Papyrus showed up one day and “asserted themselves”.

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─ In his lost soul dialogue, sans says that “you’ll never see ‘em again”.

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─ During his battle, sans reveals that he “gave up trying to go back a long time ago” and that “getting to the surface doesn’t really appeal anymore, either.”

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─ After being defeated, sans says that he’s “going to grillbys”. In DELTARUNE, the first place we see sans is at Grillby’s. sans’ house and the restaurant in DELTARUNE are the same as in Undertale (except for slight alterations like the lack of snow and "…’Sans")

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─ UNDERTALE sans and DELTARUNE sans share the exact same sprite.

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─ After talking with the Clam Girl and learning of Suzy, you can go to sans’ workshop and find a card sticking out from the back flap of the binder, described as a poorly drawn picture of three smiling people with the words “don’t forget” written in lowercase. DELTARUNE’s credits song is titled “Don’t Forget”.
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We can safely assume that sans came from another place that is not the underground nor the surface; sans misses his home and his friends there (presumably the people in the picture); Susie and Suzy are related somehow. Additionally, we can speculate that sans and Papyrus may have arrived from another dimension. It might very well be the case that sans originated from the DELTARUNE universe: sans is completely resigned to the fact that he can’t alter the past no matter how hard he tries. DELTARUNE’s prophecy is all about inevitability. sans’ defeatist attitude might stem from this.
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More debatable evidence:
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─ River Person warns the Player to “Beware of the man who came from the other world”. It’s possible that the man in question is sans. A common theory about this quote is that it refers to W.D. Gaster, however that doesn't make sense seeing as Gaster originated in and has history with the Underground and the world of Undertale in general.

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─ sans’ word search is from ICE-E’s, which is a brand we only see in DELTARUNE. As far as we know, there is no trace of this brand in the UNDERTALE universe. It definitely doesn’t appear to be one from a newspaper — it’s a distraction given to kids at restaurants.

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─ During Shyren’s encounter, if the Player hums twice, the flavor text describes sans selling tickets made of toilet paper. Now, we know that all the food in the Underground is made out of magic. Magic nourishes monsters, but it doesn’t pass through their bodies as waste. Why does sans have toilet paper?

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─ Alphys seems to be in-the-know about sans’ time-space shenanigans. In the epilogue of the pacifist ending, there’s some dialogue that suggests Alphys and sans are quite familiar with each other. Both of them want to keep this a secret for whatever reason. In a neutral ending in which Alphys becomes the ruler, she is mournful of the deaths of Undyne, Alphys, and Asgore, but is glad that “at least sans is here”. In the MTT quiz, if you pick “Don’t Know” to the question about her crush, Alphys reveals that she has done research on alternate universes and knows they exist. Both sans and Alphys have connections to Gaster, who is quite prevalent in DELTARUNE. She, along with sans (and coincidentally, the ICE-E word-search) are affected by Fun values and have their own Fun events. It seems a little random, but what if all the Fun events are Gaster-related? Are Alphys and sans the two people mentioned in Entry 17?
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Some of the basics:

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─ sans can teleport, that is, use “shortcuts”, as evidenced by appearing both in front and behind the character in Snowdin, going to Grillbys, chasing after Papyrus in the wrong direction during the Pacifist ending scene, etc. Though Rouxls Kaard is shown to teleport, Lancer’s style of teleporting most closely resembles sans’. There are multiple instances in which Lancer stays ahead despite the Player passing him (for example, Lancer sneaking a sign past the party despite there only being one way to the door).
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─ The door to sans’ room greatly resembles the fast-travel doors that only Darkners are able to create. If you get the key to his room, the pitch-black and seemingly-endless walkway is not dissimilar to that of the supply closet. Papyrus also happens to liken sans’ room to “another world”. All of the weird junk could be representative of various landmarks in the dark world.

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─ sans can manipulate shadows as seen through his first appearance (I also like to think that this is the reason why his “eye-lights” can disappear). This is most prevalent with Ralsei, Lancer, and the King obscuring parts of their body with shadows.

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─ Darkners have colored portraits. At first glance, you would assume sans doesn’t have one, but his head is already, conveniently, black and white.
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If that wasn’t enough, let’s take a look at sans’ battle in the Genocide Route. The most distinguishing feature of this fight is sans’ ability to dodge your attacks, an ability that was thought to be unique to him. As it turns out, Darkners are also able to dodge (provided that they are warned). In order to actually beat sans, you need to tire him out, and once you have dealt the final blow, he runs away. Sounds familiar, doesn’t it?

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I believe that sans actually survives the encounter as well. I’ve seen people claim that when he goes off-screen, the “dusting” sound effect signifies that he dies. This is wrong. That sound effect plays at the end of every battle, regardless of whether you spared or killed an enemy. I’d like to point out that the EXP you gain from the battle does not confirm sans’ death. If you pick on Loox, you can get 5 extra EXP stacking up to 3 times. This means that killing an enemy is not required for you to gain EXP. I’ll also include that the kill counter doesn’t go up once sans’ battle is over.

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Let me also present this tidbit that I see no one else talking about: unlike literally any monster in the underground (besides Flowey, but he doesn’t count), sans HAS NO HP BAR. When he gets hit, the damage numbers are present, but the HP bar that should have appeared above him is absent. Darkners don’t have a visible HP bar when struck.
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Stuff that’s a little bit more controversial/speculative:

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─ Lancer more or less implies that Darkners bleed, though we haven’t seen any examples of this yet. Ralsei doesn’t correct him, however. We know that the monsters in the light world don’t bleed (“Does it hurt to be made out of blood?”), and Susie’s comment (“Everybody bleeds, right?”) could be read as her trying to look cool for Lancer. Then, does sans bleed?

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─ sans’ mouth doesn’t move at all while he talks. It turns out that there may be an explanation for this! In UNDERTALE, you can see the mouths of face sprites move while words are being spoken.

─ However, in DELTARUNE, both Lightners and Darkners don’t feature moving mouths in their textboxes! In instances wherein the full-body sprites are shown (Papyrus date, Alphys date, literally all the battles), the mouths don’t move either. So while his expression rarely changes, it might turn out that sans DOES actually move his mouth while speaking, we just can’t see it. I’m a bit iffy on this theory since it varies from monster to monster in the Light World. Moreover, in UNDERTALE, while Mettaton’s mouth doesn’t move at all (I’m going to disregard this since he is a robot) it’s very peculiar that the same can be said for Alphys.

─ Seriously, why doesn’t Alphys’s mouth move at all?? This HAS to be intentional, all of the other characters with a face sprite have moving mouths. DELTARUNE Alphys doesn’t have one either. This brings up more questions than answers: Are certain monsters Lightners and others not? Is Alphys a Lightner? Did her “research” on alternate universes involve her actually traversing the multiverse? Am I overthinking this? In any case, it’s more stuff linking her to sans…

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─ If it turns out that not all monsters are Lightners, then it makes sense that sans is so devoted to Papyrus, assuming that Papyrus is, in fact, a Lightner. No other person’s death makes as much of a difference as Papyrus’s concerning sans’ behavior towards you. Alternatively, if Papyrus is not a Lightner, then it stands to reason why sans is so depressed — he has no way of fulfilling his purpose. “sans”, after all, means “without”.

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─ The Light World has virtually no indications of monsters being able to use magic. In UNDERTALE, it’s stated that magic is a method of self-expression for monsters, yet the only magic we see in DELTARUNE is from within the Dark World. If we compare DELTARUNE Toriel’s stovetop to UNDERTALE Toriel’s, we see that the flavor text has omitted her use of fire magic. Susie is able to send out magic ax attacks, but this is likely due to the nature of the Dark World. I need to clarify that she is not the one that casts the Pacify spell at the end of a “neutral” run; she realizes that the King is tired, and waits for Ralsei to finish the job. Her remembering Pacify is exactly what it means, and Ralsei comments on it because he is expressing surprise that she cared to remember the spell she made fun of him for. I’ll also point out that the lack of magic would be the reason why the monster-human war ended differently/didn’t even happen, therefore allowing monsters to reside on the surface. If sans did hail from this world, he wouldn’t have magic or would have very limited magical capabilities if he isn’t a Darkner

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─ Here is a piece of dialogue from sans about the Underground. Interestingly, he differentiates himself from monsters, though this could just be because he doesn’t want to lump in Papyrus with them. The King has a similar line (“Show my son the monster you REALLY are!”). Monsters are possibly complicit in Darkners’ imprisonment. Is it possible that sans is also resentful? Do Darkners exist in UNDERTALE?

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─ I feel like this interaction is worth mentioning. It’s entirely possible that Darkners were sealed so long ago in the Undertale universe that nobody even remembers their existence.

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Going back to Papyrus, if sans is a Darkner, then it’s important to mention that Papyrus is not. We see Papyrus turn to dust when he is killed. For that reason, it seems that sans and Papyrus are not biological brothers. I’ll also include that the kill counter doesn’t go up once sans’ battle is over. I think this actually makes a lot of sense given how different they look. Yes, it’s true that sans and Papyrus both have a skeleton motif, but they don’t look remotely similar. The face shape, the jaw, the build, the height, the eyes, the nasal cavity — nothing matches! Each monster species look the same, hell, even boss monsters like Toriel and Asgore resemble each other. Compared to Papyrus, sans seems a lot more solid (his gut) AND he’s almost completely covered up save for his face (what are you hiding under there?).

Their bone attacks are distinct as well: Papyrus’s have a more angular aesthetic compared to sans’ rounded edges.

In comparison, Toriel, Asgore, and Asriel have the same exact fire attack.

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So, if sans is a skeleton-themed Darkner, what type would he be?
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This part of the theory is a bit more conjectural, but I think there is enough evidence to put this in here.

Let’s start with how sans, Lancer, and the King are similar:

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─ Their sprites have the same color scheme: black, blue, white, and grey.

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─ They sport uneven and toothy grins, are round and chubby (“if you eat too many hot dogs…you’ll probably get huge like me”), and have a hood.

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─ The spades seem to have a smile motif. The grinning “mouth” of the Card Castle has the same number of “teeth” as sans. Additionally, you can see portraits of spade people inside the castle. They’re all smiling.

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─ These three are the only characters that can freely shift between being completely submerged in the shadows and becoming visible. The ability to cloak their body with shadows is a trait thought to be shared by all Darkners; Ralsei, after all, exhibits this all throughout our journey. What’s interesting is that there are clear parallels to sans, Lancer, and the Kings’ shadows: they conceal their entire form and reveal it during their first appearances.

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─ sans and Lancer are shown to be able to teleport. I’m bringing this up again because it seems that not all Darkners can teleport without the use of a door as indicated by Ralsei’s confusion. If Ralsei has extensive knowledge of Darkner powers, and he is unaware that Lancer can teleport, then teleportation must be an ability unique to the spades, or, at the very least, card-themed Darkners.

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─ sans and the King’s fights are eerily similar. You need to fight/tire them out and listen to them monologue in order to progress. A major theme of both fights is how you can’t truly spare them. Appearing worn out from the battle, they appeal for mercy. Should the Player fall for it, they launch an unavoidable attack that deals catastrophic damage. Additionally, before his fight, the King employs the use of silent text, just like sans during his judgments and his intimidating moments. Also worth noting: in both fights, the battle box is manipulated deliberately to suit the needs of the Player (for our last attack against sans) and the King (his weird stomach tongue latching onto the box). sans is also capable of stretching the box as seen with the attack before his special attack.

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Why the Ace of Spades:

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─ The designs of both Lancer and the King originated from kanotynes who created an entire deck of cards. The Ace of Spades does not have a distinct design.

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─ French manufacturers standardized the four playing card suits. “sans” and “Lancer” are both French words.

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─ It symbolizes uncleanliness and depression (sans’ socks and his room, the latter is pretty self-explanatory), omens (River Person’s warning, Flowey alerting us to not reveal our abilities to him, “you’re going to have a bad time”), and death (the reason for his skeletal nature).

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─ In Blackjack, Aces have a numeric value of 1 (matching his stats) or 11 (the same number of unique pre-battle dialogue from sans). The Eleventh Hour is an idiom meaning nearly too late. sans’ fight is your last possible chance at quitting before the world ends and your game gets corrupted.

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─ Remember that bit where sans has toilet paper? In the Card Castle, you can see that a bathroom, the Royal Flush, is occupied. Darkners have to digest their food. On a related note, the royal flush is a poker term for an A(14), K(13), Q(12), J(11), and 10 hand. Interestingly, in the sequence before his “special attack”, we see sans disappear and reappear at the edges of the screen 14 times

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─ The ace is paradoxically the least and most valuable card (in Blackjack, Poker, etc.), paralleling sans being the weakest and strongest enemy.

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─ Ace of Spades also represents “an end”, and sans is there at the end of our journey, judging us before Asgore’s fight. He is also responsible for initiating the phone calls at the end of every neutral route.

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─ sans was originally intended to run a casino. This one’s a bit self-explanatory. Also explains his poker face.

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The most important piece of evidence, however, is this: the French expression fagoté comme l'as de pique means “(badly) dressed like the ace of spades.” Checks out.

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On Gaster Blasters:

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─ Gaster Blasters might have a spade motif. If you connect the natural curves of the eyes and the top of the nose ridge, you get a spade shape. The GB’s crests resemble King Spade’s crown and possibly the neck/shoulder part of his cape.

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─ A GB’s nose resembles King Spade’s when he isn’t snarling at you; this might also apply to sans if he stopped smiling (mouth shape affects the shape of their noses).

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─ Also on the topic of noses, sans, Lancer, and King Spade’s noses translate to the same shape in their overworld sprites.

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─ The fog from Snowdin, which has been theorized to be either a front-facing GB or sans’ face (and honestly I lean more towards it being sans’ face because of the eye shape) has a hidden line underneath and above the nose. Perhaps sans resembles the Gaster Blasters and the Spades more than we anticipated?

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