G. Callen (nee Grisha Aleksandrovich Nikolaev-Callen) was a Special Agent in charge of the NCIS Office of Special Projects in Los Angeles.
Presumably, Grisha was born in Romania. His father was a KGB Major, and his mother was a CIA Agent. She was murdered when G. and his sister, Amy, were children. The kids were separated in foster care. Amy drowned on August 25th 1978, so they never got to reconnect.
Callen has a daughter with Anna Kolcheck, but he never got a chance to really meet her, let alone raise her.