Gwen Campbell is the third cousin of Sam Winchester and Dean Winchester. She hunts with Christian Campbell, Mark Campbell and Samuel Campbell, her cousins.
Background
She is first seen in the Campbell base when Sam brings Dean for the tour of the place and to meet the family. She also comes along when they are setting a trap for the Djinni that is attacking Sam and Dean. Later on, Sam and Dean bring a shapeshifter baby to base camp. She helps defend it when the Alpha Shapeshifter comes.
In Family Matters she is a part of a big hunt for the Alpha Vampire. Once there she get stuck with Dean, but is not too happy about it. When Dean gets suspicious of Samuel's actions, he leaves Gwen to go check on the progress of the hunt. Later in the episode, Sam and Dean talk to the Alpha Vampire, and Gwen with her cousions interupt the conversation. When the Alpha breaks loose, Gwen with the others go after it, until Crowley shows up and reveals Samuel has been working for him. This leaves Gwen questioning Samuel and everything they've done.
She continues to hunt with him, and appears again in ...And Then There Were None. On this hunt she meets up again with Sam and Dean. After an argument between Bobby and Samuel, she finds out that Samuel had given Sam and Dean to Crowley. She leaves to go ask Dean, who is outside the room. Dean tells her the truth and shoots her, due to being posessed by Eve's latest creation.