I'm working on my character as well. I'm deviating slightly from typical lore in not having him act as a pure mindless monstrosity, and not fully committed to these exact details, but my basic concept is as follows :

Ahanu is a Native American who found himself trapped in a cave-in, ate his tribe to survive & became a Wendigo. Spent the last 300+ years searching for a cure for his curse, which is how he found his way into the services of B&H. However, B&H aren't sure they want him cured due to his utility.

Tenuous alliance, as I'm imagining it.