In our Supernatural: Road to the End retrospective series, we’ll be revisiting some of the key characters and recurring guest stars who helped make the show into the fan-favorite phenomenon it became over its 15 seasons on the air before the series ends. We’ve talked to Mama Winchester herself, Samantha Smith and Felicia Day, who plays hacker-turned-hunter Charlie Bradbury. Next, we’re turning to the man behind the Winchesters’ oldest surviving (technically) ally, Bobby Singer,…