See that HEART, Kris?
That's your SOUL, the culmination of your being!
Within, it holds your WILL... your COMPASSION...
... and the FATE of the world.
The SOUL is the very essence of one's being. It plays a crucial role in the progression and plot of Deltarune. A SOUL takes on the form of a heart.
Kris's SOUL can be moved with the arrow keys/joystick. Kris's SOUL can move freely in the bullet board, and the border encloses the SOUL. In menus, the SOUL indicates the selected option, even before Kris is introduced.
During the enemy's turn in an encounter, Kris moves their SOUL to avoid attacks. If the SOUL is hit, one or multiple party members take damage depending on who the enemy targeted before their attack.
A new feature added to combat since Undertale (another game by Toby Fox with similar mechanics) is the ability to "graze" enemy attacks by remaining close to them but not being hit. When performed successfully, a heart outline around the SOUL is shown and TP is earned. Therefore, to earn TP quickly, the SOUL can be moved to risk getting close to enemy attacks deliberately.
When on the overworld, yet still in the Dark World, some enemies attack the party without engaging in battle, yet Kris's SOUL still appears to avoid said attacks. In this case, party members are only affected when Kris's SOUL collides with an attack. Grazing does not occur in these situations as no TP can be earned or used.
During the fight against King, Kris's SOUL can shine its power to a party member, allowing them to cast new and improved Spells (such as Dual Heal for Ralsei). This only occurs if a party member is ordered to Talk to King.
To leave the Dark World, Kris uses their SOUL to seal the Eastern Fountain.
At the end of Chapter 1, Kris extracts their SOUL and throws it into the birdcage found in their bedroom.