After having a look back at the announcements for the story...
I'm going to guess that he's some sort of immortal being like a demon or a vampire (not the nice sparkling kind either)
I imagine some elaborate plot to do with the princess, he was in love with her and wanted Ciel dead so he could have her for himself but she ended up escaping. So now she's going back to have a show down. Then she either thinks she kills him and he goes into hiding or banishes him for hundreds of years.
When he's finally free to roam again, the princess is long dead and he falls in love with the young girl of Ravenwood manor. So he gains the parents trust and we see how the story goes after that.
So where do we go from here?
I have no idea. Where Ravenwood Manor left off, he is probably roaming the world in search of a new victim. Maybe he's based off of something in myth?
... Too bad the princess was a lesbian. emotion_awesome
I'm only a follower of the main-plot of gaia and event plots so... pardon me for not knowing who Mephiste is in ANY sense of the word... aside from him having to do with EI's or something. sweatdrop
I do like that there is some bit of plot going on this site though, be it in the form of EI's or what-have-you. ^_^