Jonathan Farrow was a photographer and a suspected serial killer who appeared in the Season 4 episode Slack Tide.
Farrow's early life isn't specified, but it's revealed that he was arrested on suspicion of rape in New York in 2002. The charges were ultimately dropped (according to the lead investigator of the case, the victim was most likely paid off). His pictures were typically graphic, bordering on grotesque (among the ones seen was a woman eating her own intestines like noodles using chopsticks) and sometimes featured women who had really been abused by him.
Farrow first appeared on Dexter's radar when the severed arm of one of his models, Estrella Caraza, was found inside an alligator. When he was questioned, he flirted with/threatened Debra, causing Dexter to make him a higher priority than Arthur "Trinity" Mitchell. He later went to Farrow's studio to check for blood and found evidence that Caraza had been beaten there. Dexter then planned to snatch Farrow from a nightclub, but had to change his plan when Quinn appeared there. Instead, Dexter abducted Farrow from his home, garroted him to make him unconscious and took him to his studio.
Farrow insisted that he hadn't killed anyone and that he beat the women he photographed to create a kind of artistic constrast between beauty and the ugliness of the world. Despite this, Dexter killed him using a meat cleaver, dismembered him and dumped the remains into the sea. The next day, however, Angel arrested Farrow's assistant, Timothy Brand, for the murder of Caraza on air-tight evidence such as surveillance photos and DNA evidence. In spite of Farrow's mean personality, Dexter felt a lot of guilt for killing him as he hadn't actually killed someone.