The FBI informant who dies of a stroke in Season 6 is Ray Curto.
Raymond “Buffalo Ray” Curto is a veteran capo in the DiMeo crime family and one of the series’ most quietly significant informants. His cooperation with the FBI is revealed in the Season 3 episode “Proshai, Livushka,” but he remains trusted by the other mobsters because nobody in the family appears to suspect him.
Curto dies unexpectedly in 2006 during the Season 6 episode “Members Only.” He suffers a massive stroke while sitting in the car of FBI agent Robyn Sanseverino, just as he is discussing potentially damaging information about Tony Soprano. His death prevents the FBI from learning how useful the information might have been.
Curto is easy to mix up with other informants in the series. Salvatore “Big Pussy” Bonpensiero and Adriana La Cerva also cooperate with the FBI, while Jimmy Altieri is another suspected or confirmed informant. However, Curto is the one whose Season 6 storyline ends with a fatal stroke, and his secret dies with him.