Dexter and Brian

The Ice Truck Killer
The witnessing of his mother's murder instilled dark urges in Brian that drove him to kill. It's revealed that before Brian cut up his victims into pieces, he drained them of their blood. In "Seeing Red," he covered a hotel room with their collective blood in his scheme to force his brother to remember the Shipping Yard Massacre. It is unclear whether he dismembered his victims in the past, or if he started doing it as part of his plan to reconnect with Dexter.

To spark Dexter's memory, Brian left many detailed and encrypted clues. They included body parts, doll pieces, different colored fingernails, photos of Dexter's past, the blood-filled hotel room, and the actual Moser home. In the end, the brothers do reunite—but only briefly. Brian, although he was intelligent, calculating, and cautious, did not anticipate Dexter's reaction to his demand for Debra's death. ("Born Free")

In the Mask, as Rudy
Rudy Cooper is the the name of a man that Brian murdered in 1998. He then stole his identity in order to get close to Dexter. Under the alias of Rudy Cooper, Brian came off as a kind and charming man whose occupation was to help severely injured patients. According to Rudy, he became a prosthetist because his mother had lost both of her legs in an accident and he regretted his inability to help her.

Brian demonstrating that he's ambidextrous on Air Guitar

Rudy was the opposite of Dexter in outward personality. He didn't mind kicking back and having fun with others. He showed an appreciation for rock and classical music. Rudy would dance and play "air guitar," while appearing to be ambidextrous. ("Father Knows Best") The public saw Rudy as just a normal guy with no apparent issues—the perfect mask for a killer.