Keith Dillon was a Petty Officer whose murder and eventual disappearance prompted Special Agent Leroy Jethro Gibbs and his team to be called to investigate.
Having washed out of SEAL training, Dillon, feeling the need to prove himself created the alias and fictional person, "Robert J. Thorne" while carrying out contract killings.
Pryor later killed her, intending to silence her once and for all. Prior to that, Dillon's body had later appeared in a port-a-potty, giving the NCIS team a reason to continue investigating the case.
Thanks to a hypnotist, McGee was able to remember that Erin's killer had been wearing his watch on his right wrist and as such, was successful in arresting Pryor, solving the entire case.