Dr. John is a Voodoo scholar from the West Indies (Haiti) and curator of the Voodoo Museum in the French Quarter.
Background
He left his homeland and came to New Orleans for "personal reasons". Feeling that the Voodoo culture is in his blood and heritage, he is eager to preerve this culture by opening this museum. He believes that "historical Voodoo" has no relation to the "current Voodoo" of the New Orleans, nor that it is linked to the Voodoo Murders. He also performs public speeches to those who can't visit his museum.
He is happy for the opportunity to impart his extensive knowledge to Gabriel, but reveals little of his personal life. He redirects him to Magentia Moonbeam. He owns a boa constrictor which Gabriel comes to suspect is the snake present at the crime scene of Lake Pontchartrain.
On Day 5 there is a new message on the tomb of Widow Paris, that "DJ" must keeps an eye on "GK" but should not harm him. On that day the Museum is closed, and when Gabe enters, the snake attacks him. Dr. John shows up right after Gabe saves himself.
On Day 6 he participates to the St. John's Eve at St. John Bayou. Gabe arranges so that he brings a tracking device with him. If Gabe is not prepared correctly, he strangles Gabe.
When his suspicion turns out to be correct, Gabriel discovers that Dr. John is more than a Voodoo historian, but Malia Gedde's right hand man and high priest of the Voodoo cult. After Grace is kidnapped, he attempts to kill her as per Tetelo's demands, but is stopped by Mosely, who shoots him the heart dead.
Trivia
Dr. John is perhaps named after the musician Wikipedia:Dr. John, or possibly also the real "Doctor John", a Haitian Voodoo medicine man who had fascination with reptiles, after whom the musician took his name.