Humans

Humans are a race of creatures that exist on the planet Earth, and almost exclusively dwell on the Surface of the planet, but some can occasionally move to the Underground, the domain of Monsters through the Barrier entrance on Mount Ebott. Eight humans have fallen into the Underground- the First Human, the six humans that Asgore collected the souls of, and the protagonist.

Description
Humans are more 'physical' creatures than monsters, and as such are more powerful in combat. This is attributed to them being 70% water, and are therefore more concrete. A key difference between humans and monsters, due to their physicality is the continued existence of their body and soul. When a human dies, its body continues to exist (as opposed to fade into dust), and its soul also does not dissipate, which allows for collection and use of the human soul. Humans are also described as capable of living without love, to which Monsters require to an unknown degree.

History
Long ago, humans and monsters lived on the Surface together, but a war struck out between them, and humans' more impressive physicality allowed them to defeat the monsters. They cast the monsters to the Underground, and placed a Barrier around them, so that they would be unable to leave. After this, humans and monsters lived in complete separation.

In the Underground
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Humans are infamous for being wicked creatures, and after the death of the First Human and Asriel Dreemurr on the same day, their father Asgore declared that all humans that would fall into the Underground must be killed. The Royal Guard is tasked with watching out for humans, and capturing/killing them so that their soul can be brought to the king. Asgore infamously collects human souls to become strong enough to destroy the Barrier, to allow monsters to return to the Surface.

Although monsters have a negative view of humans, during a Genocide run, Flowey and Sans reject the idea that the player is a human because of their cruel behaviour; in the same run, Asgore, who fought against (and can be assumed to have seen) humans, believes the player to be another monster.

In other playthroughs, some monsters interact peacefully with the player under the assumption that they are a young monster. Other monsters know the player is a human, and may try to fight them.

Waste from human society flows through rivers into the Underground, and monsters have reverse-engineered technology from the waste. Certain items of cultural interest, such as media and former possessions have various influence on monsters; the animated medium and sequential art style of anime and manga remains one curiosity to monsters, which they can relate with the somewhat exaggerated human elements and relevant thematic qualities characteristic of the styled genre. Rarely if any direct acculturation and adoption of human civilization and social working occurs; Mettaton and Burgerpants shows examples of this through the resort, where the somewhat uptight demands and regulations of human society must be adhered to by its workers, in contrast to the lax yet responsible and complex workings of monsters.

In the Pacifist Route's ending, scenes showing life on the Surface do not directly show any humans other than the protagonist, although cities appear.

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