|Gender category:||Other (male/NB)|
|Prevalence:||Common (Western fandom), popular (non-Western countries)|
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As long as the player does not kill Papyrus, Sans is a fixture in Frisk's journey through the Underground, showing up on multiple occasions to make jokes at or prank them, and occasionally hinting that he is aware of the player's SAVE/LOAD abilities. Eventually he takes Frisk out to dinner at Mettaton's hotel, and there reveals that Toriel asked him to protect Frisk, and that if she had not done so he would have killed them on sight. He makes an attempt to persuade Frisk to stay in the Underground but then stops himself and tells Frisk that he trusts them, a line that is echoed later during Sans' segment of the true final boss battle.
If Papyrus is killed, Sans stops interacting with Frisk altogether until the judgment scene at the end of the route; in a No Mercy route he is the final boss. The Neutral route always ends with Sans leaving Frisk a phone message talking about the state of the Underground, even though the player never has the option to call Sans during gameplay.
Widespread fan lust for/fascination with Sans leads to him and Frisk being portrayed as considerably closer in fanworks than they are in-game. Stories that pair the two romantically run the gamut from portraying Sans and Frisk as each other's only allies in Groundhog Day hell, victims of Chara or the player's sadism, to enemies-to-lovers if Frisk is portrayed as at fault for manipulating the timeline.
Though Sans/Frisk is an age gap ship, how wide the gap is varies from person to person due to neither character having a definite canon age. Frisk is interpreted as anywhere from being a very young child to a teenager, and Sans from an older teen to a middle-aged adult. Some fics address the age gap: A common trope in postcanon fanwork with a romcom slant is Frisk being All Grown Up Now and attempting to make Sans treat them as an adult or viable love interest. Darkfic treats Sans/Frisk as an unhealthy relationship due to the age difference, sometimes even framing Sans as a pedophile. However, some fics, especially those where the age gap is small, treat it as a non-issue.
Due to the age gap, this ship is an absolute lightning rod for wank. The more civil end of the discussion involves larger fandom-wide issues: Fans expressing worry that younger fans might not understand what would be unhealthy about a relationship like this in real life; debate over how stringently authors should make warning tags or preface fics with thorough explanations of statutory rape for potential young readers who don't know better; and the conversation about how much responsibility fanfiction (produced mostly by marginalized independent creators with small reach for entertainment purposes) or indeed any media ought to have to educate its audience (or model only ideologically "pure" content) when parents and educators are supposed to teach children what is and isn't healthy, versus the unfortunate reality that many parents and educators fail to do so. Opinions on the subject vary widely from fan to fan.
The rowdier end of the discussion tends to involve mud-slinging, harassment, stalking, death threats and suicide baiting, and libel to the tune that anyone who likes Sans/Frisk must think that statutory rape is okay in real life, or is a pedophile or child predator in real life. Some of these aggressive anti-shippers behave this way solely because of their moral views, whereas others are upset by the ship due to personal reasons. One particular outspoken BNF's hostile comments towards underage Sans/Frisk fans have met with criticism even from fellow Sans/Frisk haters and child abuse survivors. Common pushback is that it is unfair to target young children who don't understand why age gap ships are bad, and that misuse of the term "pedophile" contributes to alarm fatigue (a la The Boy Who Cried Wolf) and belittles the experiences of survivors.
Anti-Sans/Frisk shipping wank is theorized to be a possible motivation for the attempted murder of Taiwanese fanartist Avimedes, who was fed cookies with pins baked into them and was hospitalized due to serious lacerations in her mouth. This incident sparked further fandom-wide debate on whether purity wank has gone too far or not.
For more on this general nastiness, see Purity Culture in Fandom and its related articles.
As it is theorized that some of Sans/Frisk's popularity comes from teen or child fans having crushes on Sans and using Frisk as an audience proxy, many older fans recommend writing fics that pair Sans with a reader insert character close to his age as a less contentious alternative. This is thought to be one of the reasons why Sans/Reader is so popular in Western Undertale fandom.
While this is a relationship between a nonbinary character and a male character, transphobic fans and fans who are ignorant about nonbinary identities treat Sans/Frisk as a het or slash pairing. This type of treatment is also the norm in countries like Korea or Japan where Frisk's canon gender was lost in translation.
Despite all the wank, Sans/Frisk is still the eighth most popular Undertale ship on AO3 as of July 2018. It is, however, significantly more popular in countries where there is less stigma against fictional age gap relationships.
Sans and Frisk are also popular as a gen relationship.