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 protagonist included- The First human, the six humans that Asgore collected, and Frisk.

Description
Humans are a 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. However, this excludes them from using magic, which monsters are well known for using. 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.

History
Long ago, humans and monsters lived on the Surface together, but 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
Humans are infamous for being wicked creatures, and after the death of the First Human and Asriel on the same day, Asgore Dreemurr 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.