Marx is the main antagonist as well as the final boss of Milky Way Wishes in Kirby Super Star and its remake, Kirby Super Star Ultra. He originally appears as a small jester, somewhat resembling Mirror Kirby, balancing on a multicolored ball, and he asks Kirby to help him end a quarrel between the sun and moon. Kirby listens to his plea and does as he says, unaware of Marx's ulterior motives. Marx then turns up just in time to send into motion the final part of his devious plan, and reveals that he was manipulating Kirby all along to gain power.
Upon gaining power, Marx becomes a strong, bat-like entity with flashing, multicolored "scales" on his recently-acquired wings, and endless power. After Kirby defeats Marx, he flies out of control and crashes right into Nova, causing the great machine to blow up. Despite being seemingly really destroyed in his first appearance, he manages to make infrequent cameos later in the series.
Marx makes his first appearance in the sub-game Milky Way Wishes in the game Kirby Super Star. When the sun and moon of Planet Popstar start fighting with each other, a small jester-like being named Marx tells Kirby that the only way to stop the fight between the two is to gather the power of the surrounding stars and use them to summon Nova, a planet-sized, mechanical "comet" that can grant wishes. Marx also wishes Kirby good fortune just before starting his hunt for Nova.
After gathering this power from the fountains of seven nearby planets and summoning Nova, Kirby goes on to make his wish to save Popstar. Just then, however, he gets knocked away by Marx, who then makes his own selfish wish -- complete control of Popstar. When this happens, he transforms into a fearsome bat-like monster and gains new abilities. It turns out that he actually made the sun and moon fight each other, and tricked Kirby into doing his dirty work for him, and this was all part of a master plan to conquer Popstar. Afterward, Nova begins looming towards Popstar, leaving Kirby alone in space. The power Kirby had gathered comes together, however, and creates a Starship for him to chase after them with. Nova is suddenly stopped by the sun and moon, giving Kirby the chance to enter Nova. After Kirby destroys its heart, Nova is wounded, infuriating Marx, who then attempts to attack Kirby. In spite of Marx's fearsome new powers, Kirby manages to win, sending Marx flying away and into the remains of Nova, seemingly destroying them both.
Marx's role in the DS remake, Kirby Super Star Ultra, is essentially the same as the original game, but there is a twist concerning his fate. A clip shown at the end of The True Arena reveals that he was indeed nearly killed after the events of Milky Way Wishes, but his battered body contacted a piece of Nova's power, transforming into the stronger Marx Soul and granting him even more power than ever before (Nova, on the other hand, was revived by Meta Knight in Meta Knightmare Ultra's storyline). After a series of battles against Kirby's toughest enemies, Marx Soul appears in a distorted dimension as the final foe. Denied his revenge and overthrown once more, Marx Soul lets out a bloodcurdling scream as he slowly splits in half, blowing up the arena.
The normal Marx appears in part of the Special-Edition Blooper Reel in Kirby Super Star Ultra. Marx is seen in one of the last clips when he is trying to tell Kirby how to stop the Sun and Moon from fighting, but slips off of his ball and lands on the ground before he gets a chance to talk. He also appears in the movie when Kirby gets a full 100% in a file, having a front row seat in the audience for Kirby's performance. Strangely, despite his harsh feelings towards him seen in the past sub-games, and the fact that he was defeated three times by him in this version (once in Milky Way Wishes, again in The Arena, and for the last time in The True Arena), he cheers for Kirby after he does his Victory dance.
Marx makes a surprise appearance as the final boss of Kirby Brawlball, on a board resembling that of the moon's landscape. He is much larger than he originally appeared, and used revamped versions of his original attacks. Illusions of his normal form also occasionally litter the ground, riding their balls -- bouncing on their head results in two additional Kirbys getting released.
Marx's attacks are generally very predictable but very powerful. Marx has five attacks that follow a simple, but very distinct, pattern. He starts out by teleporting himself back and forth around the arena erratically and shoots out four crescent blades immediately after doing so. He then flies up, going off the screen, and drops small seeds that grow into large, thorny vines. After the vines grow and disappear, an electrified shadow follows Kirby around the floor. After a few seconds, Marx emerges from the shadow and flies into the air, but comes back down. He then stops on the side of the screen, glows blue, and shoots a stream of arrows at Kirby. Marx flies off and then reappears soon after and spits a spiked bomb at the ground, which turns into two ice balls that slide across the screen according to Kirby's height level. Finally, Marx glows, then spits out a giant horizontal beam. The pattern starts again. After taking enough damage, his pattern will become more random and unpredictable. He will also split in half, revealing his iconic attack - the black hole. Kirby will suffer very heavy damage if he falls into the black hole. However, this is his slowest and most predictable attack, although it has a considerable range and guarding does not work either. An interesting fact about the battle is that, in the remake, after Marx has taken sufficient damage, the screen will occasionally get a red tint, possibly an effect of him straining his powers to the max in an effort to finish off Kirby.
Despite Marx's Japanese name translating closer to "Mark", the Japanese version of The Arena and Kirby Mass Attack renders it Marx.
Marx wears a hat near identical to the hat worn by Mirror Kirby, which also debuted in Kirby Super Star. The two colors on the hat are reversed on Marx's hat and the blue part on Mirror Kirby's hat is more of a teal color. This coincides with the colorful mirror-like panels that stream down Marx's wings, or him splitting in half like Mirror Kirby to perform the Black Hole move.
Marx has no upper limbs before Nova transformed him. His initial flying form has two claws on each wing, while Marx Soul has three.
Marx's laugh was changed in the remake; it was a warped sound effect in the original, but he was given a demented high-pitched laughter in the remake. In Kirby Mass Attack, his laugh is similar to the remake, but the actual board design is closer to the original version.
It is also possible that Marx had a hand behind the scenes. After 120 Energy Spheres are collected in the Extra Mode, Magolor tells Kirby that he knows someone who knew Kirby quite well, and implies that this person may have been a former enemy. Magolor also mentions that the ancients of Halcandra crafted the Lor Starcutter of "the same power that gave rise to clockwork stars that soar the cosmos," and "mysterious items that can bring dreams to life." These signs most strongly point to Marx, as these are prominent elements of Milky Way Wishes. It's also theorized to be Galacta Knight, but that character interacted more with Meta Knight than Kirby in the DS remake, plus he is more of an opponent than a true enemy.
Coincidentally, both were villains who deceived Kirby, and their use of elemental attacks is similar. The black hole attacks used by both villains have the same visual effect: if Kirby is sucked in, he is transported briefly to an alternate dimension where he takes severe damage.
Strangely, the surface of the arena that Kirby fights Marx and Marx Soul on appears to scroll rapidly from the right, even though Kirby may appear to be standing still. This is the same effect as seen in Nightmare's battle in Kirby's Adventure.