G. Callen career is an experienced NCIS agent whose penchant for undercover work is legendary. Fittingly, he was introduced during the NCIS episode "Legend."

His assignments have included stints in Serbia and Russia. Callen is described as 'a chameleon who transforms himself into whomever he needs to be to infiltrate the criminal underworld'

Twice, over the course of his career, Cullen's life was saved by Leroy Jethro Gibbs, once in Serbia and once in Russia.

Cullen is played on NCIS: Los Angeles by Chris O'Donnell.