Hráč pohybuje duší svýma šipkama/joystickem. Jeho duše se může volně pohybovat po "prostoru na duši", and the border encloses the SOUL. In menus, the SOUL indicates the 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 encounterwhere the antagonist attacks their SOUL.
If 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 back 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.
The protagonist's SOUL is red. However, SOUL Modes temporarily change its color.