Hollis Mann is a retired Lieutenant Colonel of the United States Army, formerly attached to the Criminal Investigation Division (CID), who occasionally assisted Leroy Jethro Gibbs's team with their cases. Lieutenant Colonel Mann's previous postings included the 82nd Airborne Division.
Mann became involved with Gibbs in the Season 4 episode, Sharif Returns (episode), and this relationship lasted the better part of a year, only ending when she realized how much Gibbs still mourned for his wife and daughter. She is Gibbs' only serious romantic relationship during the run of the show, and it is implied that, had it not been for his grief for Shannon, he and Mann would have married... and, most likely, stayed married. She also saved Gibbs' life in Sharif Returns after Gibbs absorbed some Nerve Gas, was overpowered by Sharif, and was almost going to die at gunpoint. Hollis then shot Sharif dead.
When her current tour of duty with CID was completed, Director Jennifer Shepard offered Lieutenant Colonel Mann a position with NCIS. She declined the offer and retired instead, relocating to Lanai, one of the Hawaiian islands.