Count Fenring was a childhood friend of the Emperor Shaddam IV, and served as an Imperial Agent on Arrakis throughout the Harkonnen’s rule of the planet. Fenring’s main claim to fame is that he’s able to make himself invisible to prescient vision, to the point where not even Paul (even after he drinks the Water of Life) can see him in his visions of the future. In the scheming political landscape of “Dune,” this is a deeply disconcerting skill. When characters like Paul and Jessica are making decisions (and counter-decisions) based on all of this, the existence of a guy who can’t be predicted tends to complicate everything far beyond anyone’s comfort zone.
Fenring is also a eunuch, which is the result of him being an unwilling part of the Bene Gesserit breeding program. He was a potential candidate for the Kwisatz Haderach, the long-prophesized figure whom the Bene Gesserit planned to make Emperor, but he was born a generation too early. His wife Margot Fenring, a Bene Gesserit witch, does appear in “Dune: Part Two,” played by Léa Seydoux — but there’s no mention of her husband.
In the book, Count Fenring serves as the interim governor on Arrakis who helps officially transition the Atreides into power. It’s not clear exactly how much he did with this role to sabotage the Atreides’ brief reign, but given his allegiance to the Emperor it’s certain that he was complicit (to at least some extent) in Leto’s assassination and the massacre of House Atreides. Physically, Fenring is described as a small weasel-like man, another duplicitous schemer in a series filled with them. The character was portrayed by Miroslav Taborsky in the 2000 “Dune” miniseries; Tim Blake Nelson doesn’t look exactly like Taborsky, but he’s a good fit for the novel’s description.