"The stars shall never shine in this universe... AGAIN!!!" (Ztarlow)
Ztarlow (also called Nega Starlow) is a Ztar Spirit who appears in Mario and Luigi: The Seven Black Diamonds. Sadly though, that is her only appearance in games so far. She may appear in other games, but this is unknown.
Ztarlow was a Ztar Spirit imprisoned by the Star Spirits in a crystal comet. Legend has it that whoever could open the comet would become Ztarlow's master. Many years later, Bowser, Bowser Jr., and the Koopalings found the comet. Bowser broke it open and became Ztarlow's master. This was revealed during the intro of her first appearance.
Her secret revealed
In one of the chapters, Ztarlow admits she is Starlow's sister, except she is darkness instead.
Her true form, Ultimate Ztarlow, is the final boss in Mario and Luigi: The Seven Black DIamonds. Ultimate Ztarlow's attacks deal massive damage, and now she can even drain some of Mario or Luigi's HP to her own