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Sans and Toriel are portrayed as close friends in-game. They first met, according to Sans, when he was using the back door to the Ruins to practice his knock-knock jokes one day and Toriel answered him. After this they met often to tell bad jokes to one another, but they never even told each other their names. Sans also says that the only reason he did not kill Frisk on sight is because Toriel made him promise to protect them.
At the end of the Pacifist route the two meet face to face for the first time and are overjoyed to finally meet in person. Toriel's estranged husband Asgore is shown looking dejected as the two hit it off, commonly interpreted by fans of the pairing as shiptease; during the postgame walkaround, the two are shown hanging out together, and if the player reads Toriel's text messages to them, is able to watch comedic back-and-forth between them. In addition, while exploring Toriel's room, the player can find bad jokes she presumably told to or was planning to tell Sans in her diary.
The Undertale Q&A event for the game's first anniversary included mention that they drink wine and eat Lunchables pizza with each other postgame.
Due to their warm friendship, many fans of the game ship them romantically, pointing to their closeness and to Asgore's reaction to watching them interact. The popular mythology AU Reapertale centers around their relationship, and Gigi D.G., creator of Cucumber Quest and one of the game's asset contributors, has gone on record to say that Sans/Toriel is her OTP.
Common tropes in Sans/Toriel fanwork include Sans' reset angst, Toriel processing the aftermath of her ended relationship with Asgore, the two characters taking solace from their individual issues in their relationship, and the two of them becoming parents to Frisk after the Pacifist route. Mundane AUs sometimes frame their romance as a long-distance or online relationship.
As of February 2018, Sans/Toriel is the ninth most popular Undertale ship on AO3, with about the same amount of posted works as rival ship Asgore/Toriel.
Sans and Toriel's relationship is also popular as a gen relationship.
- Feeling Bonely - hTeDruknenPotaT (Human AU)
- Blood of the Covenant - DoniDrabbles (also featuring Alphys/Undyne, Mettaton/Papyrus, and Frisk/Monster Kid)
- Never a Lovely So Real - Kaesa (mobster AU)
- A Monster Guide to Love - foxsgloves
- taking yourselves soriel-sly - simplycarryon
- and they recognized each other like italics - Feral Phoenix (explicit)
- Map You - Pyrasaur (explicit)
- Restless - oldandnewfirm (explicit)
- You Know What Really Gets My Goat? - Draikinator (explicit)