Kirby Super Star Ultra screenshot
|Copy ability||Paint, Ice, Cutter (from his projectiles)|
|“||He absorbed a nova's power to bring back his evil soul... This really is the last battle!”|
|— Pause screen description • Kirby Super Star Ultra|
Marx Soul is a final boss exclusively in Kirby Super Star Ultra, serving as final impediment of The True Arena. It had previously been assumed that Marx met his doom by ramming into Nova after his battle with Kirby at the end of Milky Way Wishes, but it shown in an added cutscene that Marx's body absorbed Nova's energy and grew stronger, becoming the powerful Marx Soul. It resembles the original Marx in both strategy and design, with a few aggressive additions throughout. After Kirby lands the finishing blow, Marx Soul releases a haunting scream and slowly splits down the middle; both halves then explode as the dimension grows unstable.
Marx Soul greatly resembles regular Marx in terms of his design and his battle tactics. Marx Soul's wings are now dark red as opposed to the golden-yellow ones the regular Marx has. He also has three claws instead of two. The glowing hearts on the wings are bright blue instead of red, and his eyes are now much more menacing. The pompoms on his hat are spiky and much larger, whereas in his original form they were smaller and rounder. The ends of his shoes are pointy, and he has a wider mouth with an elongated tongue and larger fangs. Marx Soul also wears a tarnished gold necklace (seemingly made from pieces of Nova) around his neck, unlike his regular form, in which he wears a red bow tie. When his full mouth and all of his teeth are shown, they are shown to be jagged and unevenly spaced apart. Strangely, the colors on Marx Soul's hat are switched. Unlike the regular Marx's hat, which has red on the right and blue on the left, Marx Soul's hat has dark magenta on the left and sky blue on the right. Marx Soul's voice is also different than the original. While the original sounds quiet and laughs a lot, the laugh that Marx Soul emits is more of a cackle that has been electronically distorted. The tips of his wings are larger as well.
Marx Soul shares most of his attacks with his living counterpart, but he has gained a few new attacks in the process of absorbing Nova's power. The most notable of these attacks is when he rains down a bunch of paint balls from the ceiling (similar to one of Drawcia Soul's attacks from Kirby: Canvas Curse); the paintballs fall slowly at first, but fall increasingly quickly. These balls will give Kirby the rare Paint ability when inhaled and swallowed, but Paint oddly does very little damage to Marx Soul. If Marx's health is about half gone, he can also split himself in two and create two huge energy blasts which move along the screen (the pink blast always goes first, with the blue blast following shortly after it is off-screen): this attack cannot be guarded against and must be avoided.
Of the attacks Marx Soul shares with his living counterpart, all of them are now significantly stronger. For example, Marx Soul uses his cutter attack two times in a row, and can move from side to side while charging up an Ice Bomb (not to mention it can be activated in mid-air, unlike Marx), making it more difficult for Kirby to evade the attack. His energy beam and arrow attacks take up more of the screen, and his black hole attack does more damage (dealing around half of Kirby's health), reaches slightly farther, is used more often, and is even Marx Soul's opening attack. Marx Soul still uses his vine attack, but now drops six seeds instead of four, and the vines that sprout from the seeds that fall from the ceiling are about twice their original size. Some parts of these vines bloom into roses, which deal more damage to Kirby than the rest of the vines. When preparing to ram Kirby from below after this attack, Marx Soul is much faster while tracking Kirby, making Kirby have more difficulty avoiding Marx when he emerges: the attack will do a small amount of damage while guarding, unlike Marx (whose version of the attack is canceled out by guarding). Finally, he also springs up more quickly than Marx, in an attempt at surprising the player.
- Marx is one of four boss creatures with "Soul" in its name, alongside Drawcia Soul, Magolor Soul and Soul of Sectonia.
- Although Marx Soul was split in half and apparently was destroyed, Marx arrives back to normal in the "Kirby Master" movie (which is unlocked when the file is 100% complete) and cheers for Kirby. This happens again in Kirby's Return to Dream Land and "Kirby: Triple Deluxe".
- In the original Kirby Super Star and its DS remake, during the cutscene where Marx makes his wish for control of Planet Popstar, the split-second before he transforms, Marx has a brief appearance somewhat resembling his Soul form (with the long tongue, sharp fangs and manic eyes).
- In Kirby Super Star, there is a glitch in The Great Cave Offense that will cause the game to trigger a battle with Marx. This time Marx looks distorted and his color will change to a purplish color, similar to that of Marx Soul. His laugh is distorted as well.
- While Marx Soul has not made a physical appearance prior or since (in favor of the original Marx), one of the rare keychains in Kirby: Triple Deluxe depicts Marx Soul.