For anyone who's played both Undertale and Toby Fox's other game Deltarune knows that the latter's cast is half made up of returning faces from the Underworld while the other half are completely new faces like Lancer, Susie, and the hilarious Queen. But it's interesting that the developer has asked fans worrying about the timeline of Deltarune in comparison to Undertale to "not worry too much about that." This article contains spoilers regarding both Deltarune and Undertale.
But it's difficult not to wonder how the two games might be connected, especially considering the many overlapping characters Deltarune has. Some players believe that Deltarune might be a prequel to Undertale, possibly set in an alternate timeline, or even both. It's undeniable that Kris bears an uncanny resemblance to Chara and Ralsei to Asriel. There's more evidence connecting the two games than just that, and perhaps the appearance of Sans in Deltarune Chapter 2 ties them both together.
Sans' Unique Abilities in Undertale
Unlike most characters in Undertale, Sans is sentient of other timelines and the SAVE function. In the game, each time Frisk dies and the player reloads a save, they are creating a new "timeline" in the game universe, allowing the player to manipulate them. Humans—and in a more meta sense, players—are stronger than monsters because they have the ability to infinitely reload save files each time they die, but the monsters in-game can't do this. Flowey is an exception to this, and he uses save files during the final fight of the Neutral Route.
Likewise, players can use the Reset function in the main menu. Because each save file acts as a new timeline, resetting effectively deletes it. Some characters will make references that the player character seems familiar to them after doing this, but Sans has some indistinct amount of knowledge of previous saves. The main evidence for this is at the end of a Genocide Route, if the player dies, he will acknowledge not only that they've died, but how many times they've died upon attempting to fight him again. Additionally, after completing a Pacifist Route once and gaining access to his room, he has a photo of him and his friends, implying he has some knowledge of the alternate timeline. However, it's widely accepted that Sans does not retain memories.
Additionally, Sans can teleport in a very unspecific fashion. He teleports away from the player character after telling them they'll "have a bad time" in Snowdin during a Genocide Route, and takes the player to Grillby's via a "shortcut," which seems to be a gimmick moving from one map to another easily.
What Sans' Appearance in Deltarune Could Mean
In Deltarune's Hometown, Sans runs a grocery store and has the same personality that he does in Undertale: pun-driven humor and a laid-back attitude. And while this might be his personality on the surface in Toby Fox's original game, his true nature is only really uncovered if players opt for the Genocide Route or kill Papyrus.
The true extent of Sans' powers has been widely theorized by fans. It's unclear how canon it is, but Sans could have the ability to jump timelines, or his knowledge of time could simply be thanks to his research alongside Alphys. But if it's true that he has some knowledge of alternate universes, then it's possible the Sans in Deltarune is aware of the Sans in Undertale.
Deltarune is out now for PC, PS4, and Switch.