A source of power to the dragon
What else would you expect from Mystery? She has a devious mind and a more sinister smile. For her, in the beginning of her existence, she was on neither side, watching, doing nothing. But the devil himself had a hand in making her mind into the way it is now. She comes off as being sexy, sweet and innocent. Of course she is anything other than that. She is powerful, a predator, and a super bitch. Mystery is in many ways a lot like the devil. Trickery enshrouds her, and the strength she has not only physically but mentally is daunting to any normal person.
She is the only thing she needs, which makes her a weapon for the good side, if they could ever find a way around her defenses. The devil uses her to his own gain, but even Mystery knows what she can do and goes out to have fun and cause misery and strife.
She remembers little of her life in the beginning; she did nothing, help no-one. Her demeanor was hard for even the gods to figure out. But Satan had to have some fun and see if he could crack her. He did many things, and only by accident, slip of the wrong words did she come out of hiding, not realizing that her silence was a curse since her beginning. And to thank him for this, she herself serves him, but only by her word and her wording always seems to have loop holes only for herself.