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.

G. Callen Quotes

(Hetty approaches Callen as he is sleeping, he wakes up staring at Hetty)
Hetty: Did you catch him this time? You were twitching like a sleeping dog.
Callen: Well, you now what they say about sleeping dogs. (looks at Hetty)

Sam: Why would you rob a bank if you already had millions of dollars?
G: It's gotta be something bigger than that.

NCIS: Los Angeles Quotes

Kensi: Just you and me. Kensi and Deeks. Deeks and Kensi. Alone we're bold, we're brash, we can move mountains. And, together we're, um,
Deeks: We're safe. Yeah I know.
Kensi: I want to be bold Deeks, but I want to be bold with you.
Deeks: So no more games then?
Kensi: No more games.

Sergeant Johnson: Cadet!
Devin: Sir!
Sergeant Johnson: What would you?
Devin: We put our lives on the line for the greater good. We don't run away from an opportunity to save others.