Humans have lived on the surface ever since they won the war. Eight humans are known to have fallen into the Underground: the First Human, the protagonist, 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 similar to those of humans.
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.