William Decker was an NCIS Special Agent.
In 1999, William Decker was part of an NCIS team that included NCIS Special Agent Leroy Jethro Gibbs and fellow Special Agent/future NCIS Director Jennifer Shepard. They were assigned the task of assassinating an arms dealer and his fiancé in Paris.
Sometime after the completion of the assignment, Decker retired from NCIS. He was later murdered with his death being disguised as a heart attack although Jenny suspected otherwise.
Decker died to protect Jenny and Gibbs, but his murder later set in motion the events of the "Judgment Day" story arc that resulted in Jenny's death too as she sought to protect her partner, Leroy Jethro Gibbs.
With his death he left some sort of "insurance policy" in the old files of NCIS. Jenny and Michael Franks figured out the coded messages which resulted with Mike taking Decker's insurance policy and has had it ever since.
It is also possible Leon Vance has some involvement with Decker's policy since he has looked the other way since.
Gibbs burned Decker's insurance policy in the season 10 finale episode, Damned If You Do (episode).