I recently dove back into the world of fanfiction, after a relatively long stint of writing only original fiction and Call of Cthulhu roleplaying games. I think the latter have had an influence on me, because the current Harry Potter fanfic I am working (Lost Souls, on fanfiction.net) is definitely a horror story. It centers on an institute that houses the soulless victims of the Dementor's Kiss as they rot away. Since this is a horror story, of course some things begin to go wrong.
Fortunately, this idea has been bouncing around in my head for nearly a year and remains strong, and I have had plenty of time to think about my characters and what is going to happen. This makes it seem less likely that I'll run into any of my traditional issues with weak characterization or losing enthusiasm halfway through. My problem, however, is that one character isn't as fleshed out as I would like him to be. He is essentially the only real human villain of the story, though others will provide obstacles (the real villains are the dementors.)
All I know about the character (Brian) so far is that he is the brother of a man who is currently inside the Institute (Wyatt). Wyatt was caught by the owners of a house he was robbing during the robbery, and in a fight that broke out he killed them. He was apprehended by the wizarding authorities and given the Dementor's kiss. Brian, his brother, has been his accomplice on multiple occasions, though through either luck or intelligence he has avoided being caught. He is ostensibly visiting the Institute to give his last farewells to his brother, but in reality he is coming to retrieve some of the loot from his brother's last robbery. Wyatt hid the goods inside him to be retrieved later with a spell--the perfect way of smuggling goods, or so he thought. Unfortunately for him, he was arrested before he could remove them. Now they are waiting there uselessly, and Brian has bills to pay...so here he comes. Later, however, when the dementors have trapped the staff and visitors inside the Institute, he becomes dangerous in his desperation.
As you can see, I have his background and role in the plot more or less sorted out. My problem is, I can't figure out what his personality should be like. Should he be a cold and calculating criminal, ready to ruthlessly do whatever it takes to get the job done? Should he be hotheaded and impetuous, ready to lose his cool at the first sign of trouble or fly into a rage over the slightest insult? I don't know what would be the most effective, and am looking for input. I don't expect anyone to be able to give me the perfect answer, as you aren't as familiar with the story as me. Still, any advice on character concepts that have worked for you, or any other ideas would be appreciated.