Human

Humans are a race that live on the surface of the Earth, beyond the barrier on Mount Ebott.

Overview
Humans used to peacefully rule over the surface of the Earth alongside monsters. However, after an unfortunate event where humans discovered that monsters could absorb their SOULs and use them for power, they declared war on the other species. After a "long battle" according to Chara, the humans eventually won the war, rounded up the remaining monsters inside a dormant volcano, and, with the help of seven of their greatest magicians, sealed them underground with a magical spell.

Humans have lived on the surface ever since they won the war. Eight humans are known to have fallen into the Underground over time: the First Human, Frisk, and the six humans whose SOULs are stored in New Home. Humans as a whole are seen as evil by monsters, especially since Asgore declared to kill any human who enters the Underground, yet most monsters do not know what a human looks like. The job of patrolling for humans is entrusted to the Royal Guard.

Humans indirectly affect the monsters' society. Waste from human society flows through rivers into the Garbage Dump. This waste has influenced monsters' technology and media; for example, anime and manga remain one curiosity to monsters. This waste has also distorted monsters' perception of human society, such as Undyne believing anime to be human history. Acculturation of human customs occurs as well, such as Mettaton's interest of keeping his resort's standards to those of humans.

Physiology
Human bodies are composed of physical matter, as opposed to monsters' more magical bodies. Because of this, human SOULs are not attuned to their bodies like monsters are. This trait allows their SOULs to persist after death. The physicality of human bodies also allows humans to carry higher concentrations of determination than a monster.

Evident from the War of Humans and Monsters, it seems that humans were once able to wield and control magic, hinting that they were not as physical and more attuned to their SOULs than they are now. Humans' current lack of magic may be because of the monsters' sudden disappearance; this suggests that monsters could very well be the source of magic on Earth, or at least to the humans.