Her voice appears to be borrowed from Red Arcueid's voice-clips in Melty Blood.
The only information known on her so far is that she appears to be a sinister and psychotic alternate-ego of Yukino, though whether or not this side of her emerged due to her latent abilities is unknown at this point. She was revealed to be one of the Final Bosses that will appear in KoF Memorial Lv2 along with Element, possibly hinting that Element may in fact be controlling her in some way to force her darker alter-ego to appear.
She does however make some appearances during some of Yukino's moves in the first and second games. In the first game, the only real mention or hinting of her is during Yukino's ultimate move in which her full power is unleashed. Yukino's hair briefly turns a ghostly white and her eyes also turn blood red during this move, the same colors as her alter-ego's hair and eyes. In the second game, Yukino is shown to have a new ultimate move in which she briefly transforms into her alter-ego to deliver a devistating attack.
Beyond this, not much of her character has been revealed. Brief seconds of gameplay footage involving her were seen in an old trailer for KoF Memorial Lv2 that seemingly revealed her to have much stronger variations of Yukino's DM's and a new Ultimate Move, though the official trailer itself has been taken down at this point.