Debuting in Kirby Super Star, Marx had the sun and moon fight in secret, and told Kirby that Galactic Nova could solve the issue. But when Kirby summoned him, Marx came in out of nowhere and wished to have complete control over Popstar! Even when gaining new superpowers and a powered-up form, Kirby had managed to defeat him and sent Marx to his doom...only to come up from time to time in cameos, even coming back as a playable character in Kirby Star Allies! (He talks about saving the galaxy from this new threat, but I think he still has ulterior motives...) For Marx's voice, try to go for a child-like voice like NESP. Around medium to high should do the trick.