Bobby, a close friend of John Winchester who served as a mentor and friend to Sam and Dean after their father's death, died during Supernatural's seventh season. He's returned a few times since then, in spirit form, most recently as part of Sam's subconscious in the Season 9 premiere.
Bobby will return for the episode titled "Inside Man." Sam and Castiel need some help from someone in heaven, and turn to Bobby for an assist.
Supernatural, "Inside Man," airs April 1.