Maenello was apprenticed to a temple of Desna as a Solarion but he found the incessant emphasis on morality and good a bit wearing on him.
That is NOT to say that he is evil. Far from it. He still has a significant element of good in him. But, if he is going to do good, he REALLY wants to be compensated for it as well. Is it really asking too much to get paid and have a good life style when one is risking ones life on a constant basis?
He has worked for a few mercenary companies in the past and now has decided to join the biggest and best of them all, the Pathfinder Society.