Arthur Mitchell, better known as the Trinity Killer, is the main antagonist of the fourth season of the Dexter TV series.
Arthur was born at some point in 1949. Not much is known about his childhood up until 1959, when 10 years old, Arthur was watching his older sister, Vera, take a shower.
Upon seeing him, Vera panicked, slipped and broke the glass screen, cutting her thigh which causes her to bleed to death. According to Arthur, both of his parents blamed him, and his mother became extremly depressed, and ultimatily killed herself by jumping off a building. Following this, his father, already a heavy drinker, began reguarly beating his son.
Ultimately, his father was bludgeoned to death with a hammer. Although it is never said who exactly commited the deed but it is very likely that it was Arthur himself. Ever since, Arthur had been recreating the deaths of his family.