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.
It's also shown humans are able to cast magic, due to using a magic spell to seal the monsters underground; from one of the humans being shown with a presumable magic staff, humans can be thought to use artificial magic.
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.
- And every time we try to change something for the better, he vetoes it and says "that's not how they do it on the surface." Oh! Right! Humans are always eating hamburgers made of SEQUINS AND GLUE. - Burgerpants
- While monsters are mostly made of magic, human beings are mostly made of water. Humans, with their physical forms, are far stronger than us. - Book in Snowdin Library
- Because they are made of magic, monsters' bodies are attuned to their SOUL. If a monster doesn't want to fight, its defenses will weaken. And the crueller the intentions of our enemies, the more their attacks will hurt us. Therefore, if a being with a powerful SOUL struck with the desire to kill... Um, let's end the chapter here... - Book in Snowdin Library
- When a human dies, its SOUL remains stable outside the body. Meanwhile, a monster's SOUL disappears near-instantly upon death. - Monster History part 7
- You see, unlike humans', monsters' bodies don't have enough... Physical matter to take those concentrations of "determination." Their bodies started to melt, and lost what physicality they had. Pretty soon, all of the test subjects had melted together into... Those. - Alphys, True Lab