John Garrett

John Garrett is a S.H.I.E.L.D. agent revealed to be both a lifelong Hydra loyalist and the mysterious "Clairvoyant" hunting the same cases as Coulson's team. His own dependency on an alien resurrection drug unravels him into violent instability, and Coulson kills him in the first season's finale.

First turns up in Agents of S.H.I.E.L.D. (Season 1).