G. Callen is a fictional character from the television series NCIS: Los Angeles. He is portrayed by actor Chris O'Donnell and serves as a Special Agent in the Office of Special Projects. Callen is known for his resourcefulness and adaptability in high-pressure situations, often taking the lead in undercover operations. Throughout the series, Callen's backstory is gradually revealed, including his mysterious past and the search for his true identity. His character is central to the show's themes of loyalty, teamwork, and the complexities of law enforcement in a national security context.