A SOUL is the very culmination of a being. Monster SOULs are upside-down white hearts, while human SOULs are normal hearts of varying colours. Different coloured SOULs have different primary traits.

Original Colour New Colour Trait Gameplay Items
Red Determination No changes since Undertale. Faulty Jetpack, Med Kit
Orange Gold Confidence No changes since Undertale. Power Glove, Fashionable Helmet
Dark Blue Light Green Co-ordination Instead of jumping, gravity is reversed. Ballet Shoes, Bedazzled Tutu
Green Black Protectiveness Instead of defending from only four directions, a shield is given which defends from a whole half of the arena, and can be rotated at a fixed speed without restriction. Plasma Pan, Stained Apron
Light Blue Cyan Courtesy No changes since Undertale. Faded Ribbon, Plastic Saber
Yellow Silver Awareness SOUL is invisible, but blinks at a fixed speed. Empty Blaster, Galaxy Cloak
Purple Magenta Precision Cards fall from the sky, and there are multiple columns. Catch the cards by pressing the buttons shown on the cards to warp SOUL into place (similar to Dance Dance Revolution but with no rhythm). Failing to catch a card pushes SOUL back, allowing for Muffet's 'pet' to take a bite. Foggy Visor, Cracked Tablet