The Golden Diva is the main antagonist and final boss of Wario Land 4. She can be fought only after unlocking the Golden Pyramid by defeating said game's other four bosses (not counting Spoiled Rotten, due to the fact that Spoiled Rotten is a joke boss) and then making it through the Golden Passage within said pyramid.
The Golden Diva first appears as a large, ghostly, floating masked woman who descends into the room in a ray of light before Wario and is then suddenly attacked by the shapeshifting black cat that had been following Wario around said Diva's pyramid(s) and helping with previous boss fights; however, said cat is unable to even slightly damage the Golden Diva and is almost-immediately disposed of by said Diva, disappearing into the nose of her mask through a tractor beam.
The Golden Diva then proceeds to cover herself with with her fan and summon four spinning masks that Wario has to throw at said fan in order to destroy it. Once said fan is destroyed, the Golden Diva attacks Wario using a collection of strange objects that are all very obvious references to at least one of the game's four main passages/bosses and must be thrown back at her and/or used against her in order to damage her (causing her to dementedly laugh as an additional result, heavily implying that she enjoys her own pain), and as she takes more and more damage, she also becomes more and more demonic in appearance.
Once her health reaches a critically low point, the Golden Diva's mask disappears completely, revealing her hideously monstrous true face as she goes into a frenzied attack, destroying portions of the floor and replacing them with spikes. After a few more hits, the monster vanishes into smoke except for its lips, which Wario extremely-anti-climactically destroys with a single blow (using his standard "shoulder charge" attack, no less) in order to free the aforementioned cat within them before then proceeding to immediately escape from the Diva's collapsing pyramid with her remaining treasure and said cat in tow. During the game's ending, said cat suddenly transforms into a beautiful (or ugly, depending on how many of the bosses' treasure chests Wario has collected) princess, having presumably been cursed by the pyramid and the Golden Diva, and gives Wario a kiss to thank him before then ascending into heaven.