You know how cartoons and whatnot often show a devil on one shoulder and an angel on the other? This kind of stems from that. There would be some romance on one side and sexual activities could come from either.
In whatever fantasy setting or era chosen, bad people die and become devils sent back to torment one hapless soul. Their goal is to turn that person bad and make the person fall in love with them by their 21st year or lead them through accident or trial over and over again until the person gives up on life. If they succeed, they may be reincarnated and return either as who they were originally, as a completely new person, or to stay with the person they were assigned to. Good people die and become angels, sent back to save the soul of a person. They do not have to make their charge fall in love with them, but it often works out that way. If they succeed, then they may be reincarnated in the same fashion as the devils. Whomever fails is either damned to an eternity in hell (the devil) or must be reassigned to another person and try again (the angel).
This particular story would focus on a young woman just turned 18. Her devil (played by one other person, character either male or female) has spent years torturing her through awful events such as the deaths of her family in a tragic car accident that she only barely survived, losing her grandparents in a housefire that only she escaped, and miscellaneous accidents that have caused her concussions and broken bones, etc. The devil also plays on her virginity, using his/her powers to give her lustful thoughts and feelings and manipulating her body without ever having to really touch it, though the devil can invade her dreams and make them reality. The girl just wouldn't realize it was reality.
Throughout all of this, the girl's angel will have been offering her unseen comfort, trying to help the girl keep her chin up. The angel could also be either male or female and would be played by one person. (I'd be the girl in question for this story.) The girl doesn't know that she's strong, but she is to be making it through all of these trials and still staying sweet and good, and the angel falls in love with her. Thus his or her goal is not only to help the girl stay good, but he/she also becomes very protective of the girl, hurting when she hurts, doing whatever he/she can to help her through all the awful things that keep happening.
Eventually I'd like the devil to lose and the angel to succeed and become reincarnated as exactly whomever he/she was whenever they revealed themself in human form to the girl. Thus the angel and the girl would then live happily ever after.
And thanks to anon who gave me the following idea I'd like to implement. Both the devil and the angel are actually friends of the girl's in human form. (At night they can be their normal form and either could invade her dreams, perhaps even sometimes battle within them if we can work that out.) These two friends could bicker, fight, pretend to like each other but really hate each other and all sorts of fun RP things, but they would know who the other is while the girl would be clueless.
So, if this sounds interesting to you and you think you'd like to play either the devil or the angel, let's talk. :)