The SOUL is the very essence of one's being, whether one is human or monster. It plays a crucial role in the progression and plot of Undertale.
See that heart? That is your SOUL, the very culmination of your being!Flowey at the beginning of the game
In the game, a human SOUL and monster SOUL together can cross the Barrier, while seven human SOULs shatter it entirely.
A SOUL takes on the form of a heart. Human SOULs point down in most cases, and monster SOULs point up.
The player moves the protagonist's SOUL with the arrow keys/joystick. The protagonist's SOUL can move freely in the Bullet Board, and the border encloses the SOUL. In menus, the SOUL indicates the selected option.
During the enemy's turn in an encounter, the protagonist moves their SOUL to avoid attacks. If, due to enough failures to do so, the protagonist's HP reaches 0, their SOUL splits and shatters, resulting in a Game Over. However, during the battle against Asriel Dreemurr, the protagonist's Determination restores their SOUL to full HP if it reaches zero. The SOUL breaks as normal, the text "But it refused" displays, and the soul fuses back together. Also, in Papyrus's battle, the SOUL can survive an attack that would reduce its HP to zero. However, it still counts as a loss since Papyrus captures the protagonist.
If the protagonist collides with hazards (such as lasers or Undyne's energy spears) in the environment, they enter a brief encounter where the antagonist attacks their SOUL.
The protagonist's SOUL is red. However, SOUL Modes temporarily change its color.
The SOUL of a monster is upside down and white. The protagonist sees monster SOULs on multiple occasions, such as when Asriel returns the SOULs to monsters' bodies after breaking the Barrier, or when Toriel or Asgore Dreemurr turn to dust and leave their SOULs behind.
A monster's SOUL is weak and does not persist after death unless said monster is a Boss Monster. Monsters believe that love, hope, and compassion compose a monster's SOUL. Human SOULs, which are much stronger, do not need any of these traits. Nearly every monster SOUL in the Underground equates to the power of one human SOUL.
A monster's SOUL composes their entire being. This is why a monster becomes weaker when an enemy gains EXP and LV, and why it becomes harder for monsters to fight when they are not inclined to do so. This trait is also why when a monster dies, their form dissipates as their SOUL disappears.
Human SOULs come in several colors and are stronger than monsters' SOULs. For this reason, human SOULs persist after death.
A human's SOUL is not the entirety of their being, but rather their essence housed in a physical body. This is why humans are stronger than monsters when it comes to physical attacks. Alphys states that determination is a component of human SOULs, and she was able to extract it from the seven fallen humans' SOULs.