Marx made his debut as a major character in the Kirby Super Star subgame Milky Way Wishes. In the subgame, Marx asks Kirby to gather the power of surrounding stars to awaken the Galactic Nova so the sun and moon can stop fighting. Once Kirby finds and awakes the Galactic Nova, Marx appears and asks it for the ability to rule Popstar. The Galactic Nova grants Marx's wish, causing him to become larger and grow crystal wings. After Kirby defeats him, Marx is sent flying into the Galactic Nova, their impact causing them to explode.
Marx appears as the final boss of Milky Way Wishes, and, subsequently, the whole of Kirby Super Star. He also appears as the final boss of The Arena subgame. In the non-canon True Arena subgame from Kirby Super Star Ultra, Marx's unconscious body absorbs the remaining energy from the destroyed Galactic Nova, transforming him into Marx Soul.
Marx's boss theme from Brawl returns as a music track.
As a boss
In Ultimate, Marx appears as a boss at the end of Kirby's, Rosalina & Luma's, and Inkling's Classic Mode run, as well as in Adventure Mode: World of Light. Normally, Vs. Marx plays during the battle, but Calamari Inkantation plays in Inkling's route, and Galeem / Dharkon plays during the boss rush. His battle takes place in the Mysterious Dimension, which is in The Dark Realm.
Marx's boss fight starts off with him using the moves that he similarly used against Kirby in Kirby Super Star:
However, Marx has moves neither seen in his original game or from his Soul incarnation in Kirby Super Star Ultra:
Upon defeat, Marx screams and starts flying around the stage sporadically. His wings break off as he reverts to his smaller form and lands face first on the ground. His body also temporarily alights in a purple flame that simmers out just before the screen fades out.
As a costume
Marx's hat is available for the Mii Fighters in the shop.
Marx also appears in the game as a spirit in both of his normal and true forms.