#2: RUDY COOPER, A.K.A. BRIAN MOSER, A.K.A. THE ICE TRUCK KILLER (CHRISTIAN CAMARGO)
Season: 1 (8 episodes)
Killer bio: Cooper, revealed to be Dexter's older brother who also witnessed their mother's chainsaw dismemberment and spent days locked in a bloody storage container unit with her corpse, grows up in a mental institution and earns the nickname the Ice Truck Killer by murdering, draining, and dismembering prostitutes, preserving their body parts for display.
What he does to Dexter: In an effort to help his younger brother recall his childhood trauma and reunite the two sociopaths, Cooper slays several hookers and leaves clues for him. Additionally, he abducts Dexter's adoptive sister, Debra, and tries to get Dexter to kill her to sever ties with his non-biological family and solidify their brotherly bond. Touching. We're tearing up just thinking about it.