Diabel only appears in the level: Old Friends where he part of the assault team of Task Force Atlas. While advancing on the street with TF Mako members: Dingo and Tick the squad got ambushed by Bosic's henchmen. Diabel ordered the squad to take cover, and shortly after the rest of TF Mako arrived to reinforce the squad. At the end of the mission Diabel can be seen alongside with Greko when Stump subdues Bosic.
"Diabeł" means "Devil", "Daemon" or "Deuce" in Polish.