Welcome to the world of Pearly Soames, where not even the characters themselves really understand what's going on.
There are two canons for Pearly, which I'm going to mix. There's the book (crazy, but brilliant), where he is the notorious feared-by-all leader of the Short Tails gang, a group of one hundred men with absolutely no morals whatsoever. And there's the movie (crazy, but mostly bad) where Pearly is still all those things, but also a demon working directly for Lucifer. The thing is, in the book such a thing is never specified, but given the way the timeline spans over a hundred years and Pearly never changes, or ages, it's obvious there's some sort of magic in play. So I'll probably go with the demon explanation, because...
The theme of the book, among others, is the balance between good and evil, light and dark. Peter Lake is the light, Pearly is the dark. So the demon explanation might not be far off after all. The New York they come from is highly magical, a living being in its own right, and possibly a place where a bridge can be built to heaven. There are miracles all over the place, everyone's connected, and crazy shit happens all the time. So for the sake of easier playing, Pearly's a demon.
SO. Things to keep in mind. Obviously there will be no IC violence without OOC permission - hit me up at writetolive at gmail dot com if you want to talk, or PM through his journal. Pearly's prone to visions at times, and also has fits where the Red Mist of Great Evil descends, and he wakes up covered in (someone else's) blood. For the most part; he can damage himself as well. He bleeds and hurts like a man, but at least in the book, it doesn't seem possible to kill him. In the movie, it only becomes possible when he chooses to become mortal in order to fight Peter Lake to the death.
He's a criminal through and through. But he is also obsessed with colour, and even seems to gain power from it - if he can be dragged away long enough to carry on with whatever he was doing before. Colour puts him in a kind of trance which he feeds off - an example given in the book has him on the way to a gang war with an army behind him, only to stop to watch workmen painting the door frame of a pub. But then he goes on, and fights like a man possessed because of it - what I'm saying is that he can be distracted with colour (or the Observation Window) if he's about to go mental, but will also be stronger when he comes out of it.
He's a viscous and skilled fighter, with a hair-trigger temper. Poke him and he'll respond. But he can also be charming - in a villainous kind of way, naturally - and he's not always going to go crazy on anyone's ass. So conversation is possible as well, and as I said, OOC interaction is love if things look like they're going to get hairy.
His jurisdiction as one of Lucifer's soldiers is limited to Manhattan in the movie, which I might bring into play to make him more easily playable. That won't change his character or personality, but might stop him trying to subvert the whole place to darkness.
Last point - his two most distinctive features are his scar, and his eyes. The scar runs up the left side of his face, from mouth to ear, and in the book is far more white and obvious than they made it in the movie - see his profile for a canon description. His eyes are impossible to miss: they are silver and shiny like electric lights, and terrifying in their emptiness. In the film they only turn like that when he's getting his demon on, but in the book they're like that all the time, so I'm going to go with it. If your pup is going to notice one thing about him, it'll be that.
Also, he speaks with an Irish accent tinged with New York.
So, come play! It'll be fun. :) Uh, at least for the muns, I hope.