What is the name of the secret boss in Kirby Super Star Ultra's Meta Knightmare mode?

The secret boss in Kirby Super Star Ultra's Meta Knightmare mode is Galacta Knight.

In Meta Knightmare Ultra, Meta Knight travels through a condensed, remixed version of several original Kirby Super Star modes. After he restores the wish-granting comet Nova, he asks to battle the greatest warrior in the galaxy. Nova responds by summoning Galacta Knight, an ancient warrior sealed away because of his immense power.

Galacta Knight is a recurring rival of Meta Knight and resembles him in several ways, including his masked appearance, swordsmanship, and use of wings. However, Galacta Knight has a pink-and-white color scheme, carries a lance and shield, and is portrayed as an even more formidable legendary fighter. He also appears in The True Arena as a penultimate boss.

A common mix-up is Marx, who is the hidden final boss of The True Arena in Kirby Super Star Ultra, not Meta Knightmare Ultra. Dark Mind belongs to Kirby & the Amazing Mirror, while Void Termina is the final boss of Kirby Star Allies.