I was thinking, either straight up darkness, like Zoran, though not as strong, and it doesn't have a mind of it's own like hers, or light, for the lols.
It just might... and... well Zoran took the insanity darkness of Giratina... xD... and Day has the uncanny ability of mastering things (It's complicated) um...how about hypnosis and possession?
She would probably end up trying to use him for power, end up getting used and losing everything but her life and Dusk, and come back to try and get revenge, and end up getting killed by him. Or heroically sacrific- ... Or die trying to get revenge. x3
^_^ It suits her. I kind of have this chibi image of her glaring at a friend's back, then when someone's harassing them, proceeds to chase the other person away with a Cacturne and laughing.