NCIS: Los Angeles' G. Callen's past is a continuous mystery, but each season has slowly revealed more about the enigmatic agent.

Who Is Leaving The 'NCIS: LA' Team? Find Out The Shocking Answer For Season 6

Hopefully, season 6 will bring the orphan closer to learning the meaning of his first initial. Like his character Callen, actor Chris O'Donnell is always hoping the information is just around the corner.

"It's been five years on the show and I still don't know," O'Donnell said on the July 2 episode of The View. "It's not me [with holding the name], it's our showrunnner Shane Brennan. It's been one of the mysteries of the show throughout the run of it, my character's search for his family and his history...I don't know. At some point I'm going to find out what the hell it stands for."

'NCIS: LA' Season 6 Premiere Date Announced, Darker Story In Store For Callen & Co?

However, Callen will be far too busy during the season 6 premiere to worry about his past. Stuck on a bomb-laden with Sam (LL Cool J), Callen will be more concerned with preserving his future during "Deep Trouble: Part 2." Meanwhile, Deeks (Eric Christian Olsen) and Kensi (Daniela Ruah) will take to the skies in an effort to scour the coast for their teammates.

Find out more when NCIS: LA returns to CBS on Sept. 29 at 10 p.m. Click the video below to see a preview of the series' latest episode.

Tags: NCIS: LA NCIS: Los Angeles television CBS