Clara Callen is the mother of main protagonist G. Callen.
Clara Callen was born in Europe as the daughter of OSS agent George Callen and his wife (a Romanian woman).
When her father was killed by the Comescu family as revenge for killing several members of the family, Clara and her mother left Romania and head to the US, where she lived until she was an adult when she started working for the CIA undercover as a student with Henrietta Lange as her handler, and during her assignment, she disappeared, re-emerging years when she made contact with Hetty, seeking asylum for her and her two children: daughter Amy Callen and young son, G. (Grisha) Callen.
While waiting for Hetty on the beach, she was assassinated on the beach by the Comescu family assassin, who was distracting her son with a toy. At the time, Callen, still very young, unknowingly suppressed the memory, later recalling the scene during an investigation into his past.
Years later, her son, Callen as an adult and an NCIS Special Agent first learned some information on her and his family. He was later sent some documents and saw her photo, causing him to start crying.