Marx Soul is a boss in the Nintendo DS game, Kirby Super Star Ultra. Marx Soul is the alternative form of Marx, who can only be battled in the final game mode, The True Arena.
Marx Soul has an alternative color from its standard form and with even more attacks than before. It can attack by trapping opponents in a dark hole, throwing crescent-like blades, planting rose thorns, emerging from a portal from underneath, throwing arrows, shooting a long-range laser, raining down paint balls in blue and pink, split itself into two while performing energy blitz charges from left and right, and throwing a bomb that creates a shock wave on the turf. Once defeated, Marx Soul lets out an ear-rapping scream before exploding completely.