This unnamed ghoul killed Adam Milligan alongside his sister and impersonated Adam.
Not much is known about this ghoul's life other than it had a father and a sister. Its father was killed by John Winchester with help from Joe Barton in 1990. After his father's death this ghoul and his sister were forced to fend for themselves and they swore revenge on the man responsible.
Nearly twenty years after the death of their father, the ghouls decided to carry out their plan for revenge. They killed everyone affiliated with John Winchester during his hunt. They killed Joe Barton, Kate Milligan, and Adam Milligan. This ghoul took on Adam's form and his sister took on Kate's form. This ghoul then called John's cell phone to complete their revenge but instead Dean Winchester picked up and informed him that John had died over two years ago. The ghouls then decided to kill Sam and Dean after learning they were John's sons. This ghoul continued to masquerade as Adam to get close to the brothers and kill them.
While Dean was off investigating one of their crime scenes Dean found a hidden room and got trapped in there by the ghouls. While there, Dean found the bodies of the real Adam and Kate. This ghoul and his sister then captured and knocked out Sam. Sam wakes up to humming, tied down to the kitchen table. He sees the Kate Ghoul nearby, cutting her fingernails with a silver knife. He realizes that silver had no effect because they aren't shapeshifters but ghouls, scavenging monsters taking the form of the last corpse consumed. The ghouls balk at the term "monster" and tell Sam that the real monster was the thing that killed their father: John Winchester.
They cut Sam and begin to drink and drain his blood as they tell him about how living off dead flesh they've survived in the twenty years since their father was killed inspired them to "move up to fresher game" and decided to start by taking revenge. "First, it was John's cop friend. And then his slut, and then his son," the Kate Ghoul says. "And I called John, but... son of a bitch was already dead," the Adam Ghoul adds. "So, I guess you and Dean'll have to do instead.", says the Adam Ghoul. The Kate Ghoul emphasizes to Sam that Adam really was his brother. As they continue to taunt Sam about how they killed Adam slowly, Dean arrives and shoots the Adam Ghoul into the wall. "Dean, they're ghouls!" Sam yells. Dean adjusts his aim and takes the Kate Ghoul's head off with the next shot, killing her. He moves to untie Sam but the Adam Ghoul grabs him and they fight. Dean finally kills him by repeatedly striking him in the head with a lamp base. Dean then unties Sam who survives the ordeal.
The real Adam was given a hunter's funeral by Sam and Dean.