Sans

Sans is a major character from the Undertale and Deltarune fandoms.

Character
Sans is the older brother of Papyrus. He is a lazy skeleton who loves making puns and drinking ketchup. While appearing as a supporting/comedic character for most of Undertale, it is eventually revealed that he has a wider role of protecting/watching over the Fallen Human after promising Toriel that he would do so. Given that Sans hates making promises, it's clear that his friendship with Toriel is what made him give her his word.

There is little known about the skele-bros past, although according to the Snowdin shopkeeper they just showed up one day and "asserted themselves". It is implied that Sans has a scientific background and is related/acquainted with a mysterious character called W.D. Gaster. Sans is also aware that other timelines exist, and acknowledges the player's ability to RESET.

Het

 * Dunkin Donuts - the ship between Sans and Muffet
 * Monikans - the ship between Sans and Monika
 * Sansdyne - the ship between Sans and Undyne
 * Sanphys - the ship between Sans and Alphys
 * Soriel - the ship between Sans and Toriel

Slash

 * Bones N Pellets - the ship between Sans and Flowey
 * Fontcest - the ship between Sans and Papyrus
 * Sansaton - the ship between Sans and Mettaton
 * Sansby - the ship between Sans and Grillby
 * Sanscest - the ship between Sans and another Sans
 * Sansgore - the ship between Sans and Asgore Dreemurr
 * Sansmaeda - the ship between Sans and Nagito Komaeda

Non-binary

 * Chans - the ship between Sans and Chara
 * Frans - the ship between Sans and Frisk

Family

 * Skelebros - the ship between Sans and Papyrus

Poly

 * Charfrans - the ship between Sans, Chara and Frisk

Cargo

 * Sanschup - the ship between Sans and ketchup

Fanon
Sans is an extremely popular character in the Undertale fandom. He is shipped with a variety of characters, including Toriel, Frisk, Grillby and fancharacters. Sans is also the character with the most AU versions and it is not uncommon to ship one AU version of Sans with another.

Toriel

Due to the characters' common like for puns and the way they look at each other in the Pacifist route, Soriel has become a popular pairing.

Fandom
FAN FICTION

List
Note: Certain ships, such as Sans/Reader, rank higher than other ships on the list, but are excluded due to the other character not being a proper character.