Callen learned his real name on NCIS: Los Angeles.
We've discovered plenty of interesting tidbits about Special Agent G. Callen's mysterious past, but there have also been questions that he couldn't answer for himself. A big one was his given name—he just knew that "Callen" is from his mother's side of the family. When he visited Russia to break Arkady out of prison in "Matryoshka, Part 2" (Season 7, Episode 16), he met a local man named Garrison. During their encounter, Garrison said that Callen's mother wanted him to know where he came from. He revealed that the "G" is short for "Grisha"—and that his full name is Grisha Aleksandrovich Nikolaev
Watch NCIS: Los Angeles on CBS.