Café de Nuit

I'm starting a new 5th ed campaign in which one of my players would like to have a dark side that manifests itself at times, specifically causing a hindrance for the party and adding to tension. He is looking to play someone who is normally a bit of a coward and one of the ideas he presented was turning into a werewolf. I know it's easy enough to infect him with Lycanthropy, but I'm trying to think about the consequences of this and what other options I could use that wouldn't over balance a game. One thought I had was the possible use of a Dark Shadow like the Lunatic (though modified) to fit the theme I'm attempting to achieve. Any other possible suggestions?

Try the "Wolfkin" background I made in "Twisted Origins":

http://mhobservatory.blogspot.com/2014/ ... igins.html

These are certainly interesting and I'll be sure to incorporate them into my game, but I'm not sure this quite fits what the player and myself are looking for.

Has he already picked a Flaw? If not, then his dark side would be a perfect fit for it.