Mike Decker was a mentor and superior of Agent Pendergast, both served together in Ghost Company.
In the opening chapters of Fever Dream (which take place in the mid-1990s), Pendergast mentions to his wife that Decker offered him a position at the FBI, which he is reluctant to accept, though he did in the wake of Helen's "death."
He was later killed by Diogenes Pendergast during the events of Dance of Death. Diogenes' plan was to kill Pendergast's closest acquaintances, while framing Pendergast himself for the murder.
Decker was killed in his home by a bayonet through the throat, moments before Pendergast discovered the body.
During the events of The Obsidian Chamber, fellow Ghost Company member and FBI agent Howard Longstreet is informed about Diogenes' involvement in Decker's death, and works to help Pendergast track him down.
However, Longstreet comes into conflict with Pendergast when Pendergast hopes to leave Diogenes alive.