-Two stoic and powerful antagonist-protagonists of 4th-wall shattering, multi-ending RPG indie games who go on a genocidal spree against a race of morally dubious creatures (the Monsters and the Spectres). Said beings are ruled by a tragic ruler, who Frisk and the Batter respectively end up killing (Asgore and the Queen)

-Their actions leads to the world these creatures inhabit to go from lively and vibrant places to empty, eeries husks seemingly devoid of life.

-At the end of the games, and after their missions are completed, they end up erasing their worlds (Frisk, whether they actually choose to do so or not, ends up erasing their world mostly thanks to Chara and the Batter turns the switch off at the end of the game, erasing his entire world).

-They are used as a foils for a higher being, the Player, who are the ones pulling the strings and are reffered to many times.

-They cross paths with a smiling judge-of-sorts who at first don't seem to oppose them, but after witnessing their mass-murders, go out to stop them (Sans and the Judge).

-Both are involved in a lot of file fuckery, hence the 4th-wall breaking nature of both games, ones which play a big role especially later in their story.