The Fox (Criminal Minds)

Karl Arnold Sr. also know as The Fox is a profiliac serial killer and the titular main antagonist of the Criminal Minds episode, The Fox and the secondary antagonist in Outfoxed.

Past and Defeat
During his childhood, he was abused by his violent father almost every day, as an adult he married a woman named Fay and became father of two children, Karl Arnold Jr. and Sarah Arnold. However, his strange behavior caused a divorce and the loss of the custody of Karl and Sarah. As a family therapist, he started to kill entire families. After investigating the case, the BAU team successfully found and arrested Karl for his crimes.

Return
Karl returned in season 5, where he helped the team with a spree-killer case. During the episode, it was revealed that the legendary serial killer know as The Reaper was secretely working with Karl Arnold.