SOUL

The soul is the very essence of ones being, be it Humans or Monsters, and plays an important role in the progression of Undertale.

A soul takes on the appearance of a heart. Human souls appear to point downwards in most cases, and monster souls always appear to point upwards.

Gameplay
The player controls their soul using the arrow keys. In menus the soul is used to show the player's selected option.

During the enemy's turn in battle, the player moves their soul to avoid attack.

If the player collides with hazards (such as lasers or Undyne's energy spears) in the environment, they enter a brief encounter where their soul is attacked.

If the player's HP reaches 0, their soul will crack and shatter, resulting in a Game Over.

The player's soul is signified with red, however its color can be temporarily changed with soul modes.

Monster Soul
The soul of a monster is always white in color and is shown in multiple occasions, such as when Flowey takes all the monsters' souls and is recreated as Asriel Dreemurr, or when you kill Toriel or Asgore Dreemurr.

The soul of a monster is weak and does not persist after death, unless said monster was what one would call a Boss Monster. Monsters believe their souls are composed of love, mercy, and compassion, but this is unlikely given that many monsters show a lack of those qualities and that human souls, which are much stronger, do not require any of those traits.

A monster is a composed wholly of their soul. This is why a monster becomes much weaker when dealing with a foe who has significant amount of hate, and why it becomes hard for them to fight when they do not feel inclined to do so. This is also why when a monster dies, they dissipate as their soul disappears.

Human Soul
The soul of a human can come in several colors, and is much more powerful than a monster's soul, which can persists long after death; as shown when Flowey absorbed six human souls and became almost godlike.

Unlike a monster, a human's soul is not the whole of their being, but rather their essence housed in a physical body. This is why a human's body persists after death, and also why humans are more powerful than monsters when it comes to physical attacks.

A human's soul is the framework in where Determination resides, though Determination can be artificially recreated, as attempted by Alphys.