Monsters

Monsters are a race in Undertale. They dwell in the Underground beneath Mount Ebott, while humans live on the surface beyond the Barrier. Every character encountered in the game besides the protagonist, the fallen human and Flowey is a monster.

Overview
King Asgore Dreemurr rules all monsters in the Underground. After The War of Humans and Monsters, monsters populated the Underground. Despite the negative implication of the word "monsters", monsters in Undertale are not an evil species. It is even said that monster SOULs are made of love, hope, and compassion.

Monsters are by far weaker than humans, both in physical combat and regarding the strength of their SOUL. This inadequacy is because monsters are mostly made of magic; which in turn makes them attuned to their SOUL. The damage a monster receives from an attacker is dependent on both the monster's will to fight and the attacker's will to hurt. Monsters, however, can obtain an "unfathomable" amount of power when they absorb the SOUL of a human.

Physiology
Monsters come in numerous shapes and sizes, although the notable species of monsters include skeletons like Sans and Papyrus, ghosts like Napstablook and their relatives, and Boss Monsters like Toriel and Asgore. Monsters SOULs are wholly white, with a couple of exceptions.

A monster's body is mostly made of magic and is attuned to its SOUL. When a monster gets old, they enter a state called "fallen down." A monster in this state lies immobile and will turn to dust shortly. When a monster dies, its body turns into dust. This dust is often spread onto the owner's things in funerals, so their essence lives on in them. Because monsters' bodies and SOULs are closely linked, when a monster dies, their SOUL is lost forever. Boss Monsters are an exception, as their SOULs exist outside their bodies for a few moments before shattering.

Because of their magical nature, monsters' bodies cannot handle high concentrations of determination. When monsters are injected with it, their bodies liquefy and melt together. The Amalgamates are an obvious example of this.

Trivia

 * It is presumed that at least 321 monsters inhabit the Underground – given the amount of monster souls visibly emanating from Asriel Dreemurr as the barrier is broken, plus Napstablook, who did not participate in the event. At least 104 monsters must be killed as part of fulfilling the requirements of the Genocide Route, plus Sans (who does not count as a killed enemy when defeated, and technically does not need to be killed to complete the route).