Dark Girl is an interdimensional being of uncut darkmatterenergy, which means she is the most powerful one of any character. She is not made of shadows, and has physical form, but fuels adulterous power with the dark. Her power weapons are dark focus, dark blades, dark vision (which can usually be dark lazer), and dark taunt. She also has control over ninjas, bats, monsters, spiders, and any other things that live in the DARK. She is a most culpable fighter and can hand-to-hand with anyone.
She is not related to the Dark Queen, but in their fuedal system had to be adopted by the Dark Queen because of the consolodation of power, both in a Romanesqe political sense, and as in power level as when the electric company overcharges you and all the roommates have to split it up. She already had parents of her own, so the Dark Queen had them killed so it looked like a completely different murderer had killed them by accident.
Dark Girl lived for 500 years, so when she met Breshvic in another dimension they had a brief affair, it was awkward because by the DARK Dimension's social standard, 500 is still way underage. And in Earth's dimension, Breshvic was not even born yet! The Dark Queen tried to have Breshvic killed by her minions, but he was too powerful. Finally she tried to kill him herself but they blew up an entire planet and had to go their separate ways. Breshvic dedicated a Journey song with that title to Dark Girl, because he believed he was in love with her. She felt it too because the Dark Queen's spell had worn off and now she hated her adoptive mother as always and wanted to rebel and run away with him. But once she did, she realized that she wasn't really in love, she just wanted what she couldn't have. She left him on an asteroid and has been one of his sworn enemies ever since.
It was revealed in an issue of FUTURE TALES that Dark Girl does indeed become the Dark Queen as foretold in the legends, and that is why she hates the Revengerists so much, forgetting all the good times and dwelling only on the awful betrayal. This is why she adopted herself because it turns out they really are related after all, I just had not read that part yet when I put it in here the first time. UPDATE: It turns out they were NOT related but in an alternate timeline and that was retconned so it is original canon now. So in fact it was always Dark Girl who adopted the Dark Queen but in reverse time so the chronology is now forwards instead of before the timeline realignment. In this new original version, the betrayal was always done by the Dark Queen, who wanted to marry her adoptive daughter off to another evil Dark Revengerist of the Vengeance named Dark Fedora but Breshvic defeats Ze Black Waffle during a Timequake and Dark Girl had basically fucked off to her own limited series by this point, making the entire arc meaningless.