Ah, Necanthrope Jekyll. I've been meaning to get around to creating him as an actual character, and then using him as a regular NPC in a gaming situation.
Lets see... His personality is one of someone fighting for control. While his Hyde persona is a Soul Weave, he also divides his emotions quite clearly between his two minds. Jekyll is a creature of fear and knowledge. He is one of the greatest minds in Dark Lament, and has been accumulating information about all sorts of subjects for centuries. Hyde, on the other hand, is a very violent creature, letting his emotions carry him in whatever direction they may. However, Jekyll is the one who is in control (most of the time) and so his incarnation would be one of subservience. IE: Everyone in his presence feels in awe of his knowledge and power, and considers themselves to be beneath him. Game use would be something like a concentration roll at minus 5 or they will have to roleplay the most crawly and slimy character they can think of or have mental disadvantages added to their character. Or something.
Jekyll seems very old, a little like Yoda in his face, but tall and gangly, and without the silly ears. He is vaguely reminicent of a daddy-long legs, his spindly arms bending far too far. He looks as if he would break if he moved too quickly.
If you want them to encounter Hyde (which could be fun, if they build up a picture of Jekyll, and then find this firey violent Nec in his place) His incarnation would be one of blind, unthinkable rage. Failing a conc roll at -5 would force a player to attack the closest person to him, and attempt to kill his victim as best he can with his bare hands. Physically, Hyde becomes a more fearsome character. His deathsuit bulks up, changing the image of a tall, wirey man to a giant, capable of snapping you in two like a twig. Hyde is much more agressive, and will shout and scream at the players should they try and defy him.
Maybe it would be best to have the players avoid Hyde, actually! :-)