Arthur Mitchell aka The Trinity Killer is a character and the main antagonist in Season Four of the Showtime series DEXTER. He also briefly appears in Dexter: New Blood in a podcast photo ("H is for Hero") and in flashbacks ("Skin of Her Teeth" and The Family Business).
Summary[]
Several years after Arthur's disappearance, Molly Park, a true crime podcaster from Los Angeles, covers the story of the Trinity Killer in an episode of her podcast. A now teenage Harrison, having recently reunited with Dexter in Iron Lake, listens to the podcast alone in the school gymnasium and grows unstable hearing the details of his mother's death. Soon after, a lockdown occurs at the high school, and Dexter is notified that Harrison was involved in an injury. Arriving at the school, Dexter confronts a wounded Harrison and the police. Harrison states that his friend, Ethan, had asked for his help in murdering several of their classmates, to which he denied, and was subsequently attacked with a knife. Harrison, with a minor stab wound in the abdomen, says he slashed Ethan's leg in an act of self defense, potentially to mimic Trinity cutting Rita's femoral artery. Later, suspecting that Harrison may be lying, Dexter finds a straight-edged razor identical to the one Trinity committed his bathtub murders with. He now realizes that his son has a Dark Passenger as well.
After attacking a group of rival school students with his straight-edged razor, Harrison is stopped by Dexter. He accuses Dexter of thinking he is a 'psycho' and that is the reason he left him. He then admits to him that he had had nightmares of his mother dying when he was young, but only realized they were true after hearing Molly's podcast about the gruesome event. In a flashback to their old home in Miami as Arthur climbs out of the bathtub of after murdering Rita, then approaching a crying infant Harrison, gently telling him, "Daddy will be home soon."
Following Dexter's confession to Harrison that he has violent tendencies identical to his due to him also witnessing his own mother's death, Harrison explains his rage toward Trinity, and confesses that he has fantasized of finding and killing him with his own razor. Later, in Kurt's bunker, the two discover Molly's corpse, setting off Harrison and leading him to an epiphany; Dexter murdered Wiggles, thus stopping his reign of terror. Dexter confirms that he did, and reveals to him that he killed Arthur as well, though he lies and claims it was in revenge for Rita's death, rather than admitting that he had not known she had died until after he killed Arthur. Harrison claims that Kurt must die too.