Thanks for all the ideas, I got some good stuff to work with. Lets try first build.
The killer doesn't know he's doing it, or that he's a changeling. He lives a life that is so normal that his mystic side takes over at times. The killer doesn't notice that sometimes he's missing a couple of hours and the fae side acts out its troll/ogre/gruff/goblin/whatever impulses when its in charge. Do the PCs kill an innocent man just to stop the killer?
Name: Joe
High Concept: Half-Goblin Who Doesn't Know It
Trouble: Killer Inside
OA:(Human Side)
Bank Teller
One of the Nicest Guys You Could Ever Meet
Squeamish
(Fairy Side)
Time To Play
Can't Let Joe Catch Me
How Come He's Not Changing Into Me?
Joe was adopted at a very young age, he doesn't know anything about his parents or the supernatural world at all. He was a little mischievous as a kid, but his adopted parents were quite strict with him and he pushed that side of him down.
As an adult, his fairy side finally wants a chance to come out and play. When Joe is sleeping, bored, or otherwise has his guard down the Goblin inside can rise to the top to play for a while. When ever the Goblin is in control it uses it's fairy powers as much as possible hoping to choose for Joe, but only Joe can choose. Because Joe has never made the choice to use the powers, his mortal side shows no signs of being fairy.
The Goblin doesn't want to get caught, it is having way too much fun. The only real pattern to it's victims is that they are all crimes of opportunity. Girl walking down the street at night, guy sneaking around stalking someone, etc... The problem is that the couple of people who have gotten away have offered completely different descriptions of their attacker. The authorities refuse to call this a serial killer because of the lack of anything connecting the different cases, but people on the streets of Boston are getting scared.
Supernatural Powers:
Glamours [-2]
Claws [-1]
Supernatural Strength [-2]
Supernatural Speed [-2]
Supernatural Toughness [-2] catch is: Cold Iron and trappings of Summer