UNDERFELL FRISK

lyf1234

You're right,but Underfell is an AU of the well-known role-playing video game Undertale where all of the good-natured monsters who don't actually want to hurt the protagonist, are replaced by vicious versions with opposite intentions and vice versa. One prime example of this is Flowey, who, normally shown as being an antagonist in Undertale, actually wants to help the protagonist escape the Underground safely. It presents a universe where the Underground monsters are darker versions of their originals. Some fans refer to these characters with 'Nega' but must refer the characters as 'Fell' as a prefix to differentiate from canon.

There are many interpretations of this universe, but most share similarities such as Frisk falling from Mt. Ebott and meeting Flowey, who isn't evil but rather a guide through the Ruins, and instead, King Asgore is the one that has instated the motto "Kill or be killed" and the one who poisoned Chara rather than themselves.

Underfell was foremost created as a design aesthetic, so there originally wasn't a canon backstory to the characters outside of the idea that they are an alternative, evil version of the regular Undertale canon characters. However, there are some various ideas about the characters that are widely accepted, such as Sans getting easily flustered and irritated by flirting, puns, and having an obsession with mustard or Papyrus being a cold individual with no patience for failure (especially not from his brother). Later on when the creator of Underfell returned actual canon personalities got created.

All of the monsters are essentially malicious/more violent than in Undertale. They attack one another and don't all appreciate the fact that Frisk is befriending them. Instead of promising to protect the human, Sans is dared to kill them. Instead of the wide arrange or friendly colors, most characters tend to share a fashion of black, red, and gold, with red also being the color often used for battle sprites. Flowey and Mad (Sad) Dummy are pretty much the nicest characters/only nice characters (and some of the only monsters without black, red and/or gold in their character design). Monsters weren't always this way however, their evil nature is caused by the downward spiral the Underground got into.

Even though the creator returned to the community, the AU still remains community-shared however as they have gone on to continuously state they are in full support of all non-problematic takes and interpretations,very much enjoying them and asking fans not to send hate to AU takes and designs just for going outside Fella's personal interpretations.
（上面这段话来自UNDERTALE AU WIKI）