SOUL

The SOUL is the very essence of one's being, whether they are human or monster. It plays a crucial role in the progression and plot of Undertale.

A SOUL takes on the form of a heart. Human SOULs appear to point downward in most cases, and Monster SOULs always seem to look upwards. The possible exception might be Mettaton, whose SOUL points down, but it is not confirmed if the heart that appears in his battle is actually his SOUL.

Gameplay
The protagonist's SOUL is moved with the arrow keys/joystick. They can move their SOUL around freely in the Bullet Board, although they are enclosed within its border. In menus, the SOUL is used to show the protagonist's selected option.

During the enemy's turn in battle, the protagonist moves their SOUL to avoid attacks. If the protagonist 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 protagonist's HP reaches 0, their SOUL will split and shatter, resulting in a Game Over.

However, during the true final battle against Asriel Dreemurr, Frisk's Determination restores their SOUL back to full HP if it reaches zero. The SOUL breaks as normal, "but it refused," and fuses back together and stays alive during the fight.

The protagonist's SOUL is signified with red. However, its color can be temporarily changed with SOUL Modes.

Monster SOULs
The SOUL of a monster is always white in color, upside down, and is shown on multiple occasions, such as when Flowey takes all the Monsters' SOULs and is recreated as Asriel Dreemurr, or when Toriel or Asgore Dreemurr are dying.

A monster's SOUL is weak and does not persist after death, unless said monster was what one would call a Boss Monster. Monsters believe that a SOUL is composed of love, mercy, and compassion, but this is put into question because human SOULs, which are much stronger, do not require any of those traits. It is said that the culmination of nearly all the monster SOULs in the Underground is equivalent in power to one human SOUL (which may be a fact combated by certain points of the game).

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

Human SOULs
The SOUL of a human can come in several colors, and we see a total of six (Red, orange, yellow, green, light blue/teal, blue and purple) Human souls are much stronger than a monster's SOUL, persisting long after death. The color is shown when Flowey absorbs six human SOULs and becomes almost godlike. Although not quite, since he only needed one more. All of the monsters' souls combined would simply equal a single human soul.

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

A human's SOUL is the framework in which Determination resides though it can be extracted from human SOULs, as attempted by Alphys which resulted in the creation of the Amalgamates.

Душа SEELE ALMA ‎DUSZA SOUL