Marx Soul is the final boss of Kirby Super Star Ultra, serving as final impediment of The True Arena. It had previously been assumed that Marx had met his doom by ramming into Nova after his battle with Kirby at the end of Milky Way Wishes, but it was shown in an added scene that Marx's body absorbed Nova's energy and grew stronger, becoming the powerful Marx Soul. He resembles the original Marx in both strategy and design, but with a few aggressive additions throughout. After Kirby lands the finishing blow on him, Marx Soul releases a blood-curdling scream as he slowly splits down the middle and turns purple; both halves then explode as the dimension grows unstable.
Marx Soul greatly resembles regular Marx in terms of his design and his battle tactics. His wings are now dark red, have three claws on each wing, the hearts on them are bright blue, and the tips of the wings are larger as well. His eyes are now much more menacing, where he has a tiny pupil surrounded by a single thin lavender-colored iris. The pompoms on his hat are large and have spikes on the top, whereas in his original form they were small and round. The ends of his shoes are pointy, his mouth is wider with an elongated tongue that hangs out of it like a dog, and larger fangs. Marx Soul also wears a tarnished gold necklace around his neck. When his full mouth and all of his teeth are shown, they are revealed to be jagged and unevenly spaced apart. Strangely, the colors on Marx Soul's hat have been switched, having dark magenta on the left side and sky blue on the right side, unlike the usual sky blue on the left side and dark magenta on the right side. Marx Soul also has a laugh that sounds like a cackle that has been electronically distorted.
Marx Soul shares most of the attacks he uses with his living counterpart, but he has gained two new attacks in the process of absorbing Nova's power which involve him splitting himself in half. One of the two attacks Marx Soul performs involves him raining down pink and blue paint balls from the ceiling (similar to one of Drawcia Soul's attacks from Kirby: Canvas Curse); the paint balls fall slowly at first, but soon start to fall increasingly quickly, making it nearly impossible to dodge them all. The best way to take the least amount of damage is for Kirby to move all the way to the side of the screen and guard himself. These balls will give Kirby the Paintability when inhaled and swallowed, but Paint oddly does very little damage to Marx Soul. The second of the two new attacks Marx Soul performs only happens when his health is about half gone. At this point, he splits into two energy orbs (one pink and the other blue) which move along the screen, in which the pink blast goes first, with the blue blast following shortly after it is off-screen, although it's mixed at times. Also, none of his attacks can be guarded against now, 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 performs his cutter attacktwice rather than once, and the attack's name has been changed from Quad Cutter to Octo Cutter.
Marx Soul moves from side to side while charging up the ice bomb in his mouth.
Marx Soul now drops six seeds instead of four. The vines that grow from the seeds are twice their original size, and some parts of them bloom into roses, causing more damage to Kirby should he be hit by them.
Marx Soul is faster and more precise when tracking Kirby from underground, and he springs up much faster and much higher than he does in his normal form.
The arrow attack and laser beam take up more space on the screen and deal more damage.
In comparison to normal Marx' version, Marx Soul's black hole attack deals more damage, has a wider range, and is used more often. Unlike Marx, he even uses his black hole attack as his opening move.
Marx is a character in Kirby Super Star and Kirby Super Star Ultra. He was a diminuitive jester of Dream Land with big plans. He made the Sun and Moon of Dream Land to fight with each other, and asked Kirby to summon Nova using the powers of the Dream Fountains of surrounding planets and wish the fighting to stop, but he has his own plans for that one wish Nova can grant - complete control over Dream Land.
Marx Soul's Pause Screen description first confirmed the existence of multiple Novas, as it says "a nova's power" as opposed to "Nova's power." This is one of three sources indicating this, along with Magolor's mention of the Clockwork Stars and the appearance of Star Dream.
Although Marx was split in half and supposedly put down for good, he arrives in his normal form as if nothing happened in the "Kirby Master" movie (which is unlocked when the file is 100% complete) and cheers for Kirby despite him being defeated by him (three times if the remake and the non-canon True Arena are included). This happens again in Kirby's Return to Dream Land (similarly to Magolor) and Kirby: Triple Deluxe, and in Kirby Star Allies where he is a playable character.
On the other hand, the Kirby Master video may not be canon, though this doesn't eliminate the possibility of this happening if Kirby defeated Marx Soul.
In the original Kirby Super Star and its Nintendo DS remake, during the cut-scene 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, sporting a long tongue, sharp fangs, and slit pupils. This is more noticeable in the remake, and possibly what inspired the design for Marx Soul.
A rare glitch within Kirby Super Star allows one to trigger a fight against Marx outside of his intended battles within Milky Way Wishes and The Arena. When triggered within The Great Cave Offensive, Marx would look distorted while his color changes to a purplish color, similar to that of Marx Soul. His laugh is also distorted as well.
While Marx Soul has not made a physical appearance prior or since Kirby Super Star Ultra (in favor of the original Marx), one of the rare keychains in Kirby: Triple Deluxe depicts Marx Soul splitting himself in half.
While Marx's arena background begins to pulse red and green once he's low on HP, Marx Soul's battle arena stays the same throughout the battle.
Marx Soul is the only soul boss so far to have a unique battle arena, whereas other soul bosses have the same arena as their original selves. Star Dream Soul OS’s fourth phase is debatable, as it uses a heavily modified version of the arena for President Haltmann and the final bosses of Meta Knightmare Returns.
Marx Soul, Magolor Soul and Void Soul are the only Soul bosses so far to share battle themes with their original selves.