Marx is a recurring character who made his debut in Kirby Super Star as the main antagonist. Kirby and his friends meet him at the beginning of Milky Way Wishes, and he asks if they can solve a conflict between the Sun and the Moon. Kirby and his friends agree, unaware that Marx is planning on using their aid for his own evil ends. Marx then steals their wish and makes a wish on the Galactic NOVA to control Kirby's home planet, transforming into his True form. Kirby then obtains the Starship ability and destroys the Galactic NOVA's heart, stopping his movement dead. The pink hero then fights and defeats Marx, sending him flying into the Galactic NOVA, their collision seemingly killing them both as their bodies appear explode upon impact.
Marx is also the final boss of the Pinball eqsue Kirby Brawlball and as a playable character/Boss(in the credits) of Star Allies.
In Ultimate, Marx appears as a boss at the end of Kirby, Rosalina & Luma, and Inkling's Classic Mode run, as well as in Adventure mode, as a servant of Dharkon. He resides in the Mysterious Dimension area. When the player answers the question about the Subspace Emissary correctly, Marx will appear in the center of the swirl and pull the fighter to his space. However, the player can leave this area and is not immediately forced to fight him. Marx voiceclips are similar to those in Star Allies.
In addition to his boss appearance, Marx’s regular form appears in the game as a spirit. When he is leveled up all the way, he becomes his True form.
Marx keeps most of his attacks from Super Star such as teleporting, shooting crescent blades, dropping an icy bomb, creating a black hole, and spitting out a massive laser beam. However, he has a new arsenal of attacks such as eye lasers and appendages that deal flame-type damage. The arrow attack is unused.
Marx also appears as one of the many different bosses fought in Sephiroth's Classic Mode along with every other boss (excluding Galeem and Dharkon)
When entering Marx’s battle, it opens into an area reminiscent of Planet Pop Star, the area in which Marx is fought in Kirby Super Star. The screen turns to looking forward and no one is there. The camera then turns to look behind the fighter as they stop moving. Marx is just standing there. After a few seconds, Marx floats in the air and then transforms into his True Form, starting the battle.
When defeated, 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.
All move names are derived from the Japanese version'
- 4 Way Cutter Attack/Shooter Cutter(JPN): Shoots out 4 boomerangs that work similarly to Kirby's Cutter ability, which then return back to him. Like his Soul form, when he is at low health he will do this move twice. They can be reflected or consumed by Inhale or Chomp.
- Extra Thick Laser/marx Cannon: Teleports to the players' y coordinate and fires a laser that covers the stage. Unshieldable, but can be armored and, like in Super Star (Ultra), can be ducked under.
- Ice Bomb/Ice Bowl: Teleports to the top of the stage, then spits out an Ice ball, which splits into 2 icy, freezing shockwaves when reaching the player's y coordinate. PSI Magnet and Absorbing Vortex can absorb them.
- Makai Plant/Seed Attack: Flies offscreen and drops 5-7 seeds that grow into thorny vines. Like Shooter Cutter, the attack changes when at Low Hp, they will grow faster and appear in larger quantities. Unlike his source games, shielding will not destroy the seeds.
- Jump Out of The Ground/Shadow upper: Like Shadow Sneak, Marx appears out of a shadow from wherever the player is. Marx often uses this attack when he is offscreen, and will do so more ofetn at low HP.
- Black Hole: Marx splits into 2 and opens a Black Hole that will try consume the player. If the player gets caught, they will take massive damage and get spiked into the ground. This attack can be predicted, as Marx will foreshadow it k with three teleports from side to side, until stopping at the exact center of the screen. Marx is can be damaged prior to splitting but is intangible afterwards.
- Tracking Eye: Marx's eyes go completely black, then black balls come out of them and start bouncing around the stage, dealing darkness damage to the player if they hit. At low health, Marx will spawn more black balls, up to 6.
- Capillary Expansion: Moves to the middle of the screen and retracts his wings, where pink, branching appendages resembling blood vessels grow out instead, spreading across the screen and dealing fire damage to the player. The attack is vertically mirrored. There are two variations: one in which the appendages fill the center bottom of the screen while leaving the corners untouched and vice-versa.
- Compound Eye Laser: Grows his eyes into bug like compound eyes that fire lasers(which increase in quantity at low HP) at the player, dealing electric damage
- The Black Hole Marx creates is similar to the Big Rip move Crazy Hand can perform in Super Smash Bros. 4.
- Along with Dharkon, Marx is the only boss in Super Smash Bros. Ultimate who was not revealed in the game’s promotional material as both Giga Bowser and Ganon were shown as Final Smashes in their character’s spotlight trailers, Rathalos was first announced as an Assist Trophy, Dracula appeared in the Belmont reveal trailer (only his first form though), and both Galeem & Galleom were revealed in the World of Light trailer.
- According to Sakurai in his Sephiroth presentation, Marx was originally going to look creepier, but was toned down presumably to keep the game's rating at E10+.
- For some reason, Marx is the only boss to be fought in World of Light that does not have an area themed after his game of origin: His area appears to be a destroyed variant of various locales in the mode, as opposed to Planet Popstar's moon, on which he is fought in Kirby Super Star. His boss arena appears visually similar to its original appearance, however.
- Marx has the smallest hurtbox of all the bosses, even smaller than Dracula's head hitbox and even smaller than some of his foes.