Seth is a drug dealer turned power dealer. Before the storm he was your run of the mill drug dealer, albeit a successful one. After the storm he had the ability to take and give people powers, for a price. The group sells their powers to him, starting with alisha (for obvious reasons). Later when their powers are being abused by the person who bought them from Seth they buy back different powers. During the third season Kelly becomes more and more interested in Seth and we find out that he believes he is responsible for his girlfriend's overdose (on the drugs that he sold her).