Xevenita Dark X Shine

Xevenita Dark X Shine is the lost Princess of Darkness. Cloned from the original Master Xehanort's darkness, she took on the appearance of his feelings towards darkness combined with his own appearance since she is a clone. Plus since she was born from his strong feelings, she is extremely super powerful and could destroy entire worlds. Xehanort couldn't turn her into a vessel because she was really self aware and he could not control her. So he tricked her into thinking she would be, but instead he sent her to Tartarus where Hades watches over her and keeps her subdued because he is a god and Xehanort made a good offer to him.

But after a few of the people Sora and Riku killed went to the Underworld, she got word of Xehanort coming back, so now she will come back as well once she figures out how to get out of Tartarus.

Keyblade
Xevenita Dark X Shine's Keyblade, Umbra Edge, is half real and half fake. Since she is only Xehanort's darkness and feelings, she can not make a full, real keyblade. So she took what she had and fused it with real steel to make Umbra Edge. It is also full of darkness so when she hits people, it infects them with darkness, turning them into her slaves. The edges make them bleed out their light, so the person will be 100% Darkness.

She holds it in a reverse grip like Ventus.

Story
Xevenita Dark X Shine was originonally going to be Master Xehanort's vessel and pupil so she could spread darkness. But since he got rid of her, she has been with Kronus and some other Titans while waiting to be set free. Since that has been her only company, she marries Ice Titan and together they happily live with the others plus other Final Fantasy villians who could not just be put in the normal part of the Underworld. Like Kefka, Cloud of Darkness, and Orphanus.