Marx is a non-playable boss character in the fighting video game, M.U.G.E.N Trilogy. He was confirmed along with the other bosses on July 4, 2018.
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.
In addition to these appearances, Marx appears as the final boss in the Kirby Mass Attack subgame Kirby Brawlball, and as both a playable character and a secret boss in Kirby Star Allies.