G. Callen

G. Callen is the main protagonist of the CBS series NCIS: Los Angeles, which is a spin-off of the original NCIS. He is portrayed by American actor Chris O'Donnell.

Callen is the NCIS Special Agent in Charge of the NCIS Office of Special Projects team stationed in Los Angeles, reporting directly to the OSP Operations Manager Hetty Lange. He and his friends/teammates were introduced in a backdoor pilot, "Legend" from NCIS. He and Lange first met three years prior to the start of the series, as revealed in the season one episode "Identity".

While his friends address him as "G", Lange refuses to do so, saying, "It's not a name, it's a letter." She instead addresses him as "Mr. Callen" ("Identity"). Callen once served with his good friend Leroy Jethro Gibbs, who now works at the NCIS Headquarters located in the Washington Navy Yard. Callen specializes in deep undercover work.