Seth

Seth was a vampire who teamed up with Victor Rogers. Victor had given him families to target and kill, so that Victor can take in the surviving children and raise them as hunters. He recruited Krissy, Aiden and Josephine Barnes. Seth then turns new people into vampires, and Victor convinces the kids these are the vampires who killed their families, so they can get hunting experience and become better.

Season 8
Seth, driving a blue van, drops off a new vampire near the hotel, he knows Aidan, Krissy and Josephine will be waiting and waits to see them kill it. Next he kidnaps a local woman named Connie, and takes them to a motel. He brings Jimmy Day to her, but he is pursued by the kids, and also Sam and Dean. He is killed by Josephine as Seth watches from his van. Connie later tells Dean that it was a man in a hoodie driving a blue van who took her, not Jimmy.

Later Sam observes a blue van outside the house where Victor and the kids live. He tells Victor but when they go in pursuit, Seth knocks Sam unconscious. Returning to Victor's house, the group finds Sam tied up by Victor and Seth, who had planned to kill Sam and make it look like a vampire kill, in order to further motivate the kids to hunt. Sam and Dean reveal what they have figured out: Victor has had the vampire kill their families to drive them into hunting so he could train the next generation of hunters and having them get "revenge" by killing people who have just been turned and he has framed. When Krissy asks who the real killer is, the vampire informs her that he killed her father and enjoyed it. Victor disarms himself and tries to convince the group to keep going with him, but they refuse and he has the vampire take Aiden as a hostage so he can escape.

Krissy shoots the vampire in the right eye and chest with darts filled with Dead Man's Blood.