Sergei Mishnev was an Israeli-born mercenary.
The son of a Russian officer, Mishnev's father, Dimitri presumably had an affair with a Palestinian woman, Dr. Hasmia Haswari, who was also a Saudi national and the mother of terrorist Ari Haswari while Dimitri himself was stationed in Israel.
As a result, Sergei was born in 1965 and as such, was the maternal half-brother of Ari due to the fact that they shared the same mother but had different fathers.
Much of Mishnev's past remains unknown but when Sergei was ten years old, his father took young Sergei back to Russia, leaving Sergei's mother as well as Ari in Israel.
Years later, Mishnev attended Soviet university where he met the future Russian Councilor Anton Pavlenko and worked on a sport with him.
After they failed in the event, Sergei parted ways, going for an Afghani opium cartel.
At some point, Sergei also presumably formed an alliance with numerous terrorist groups. He also presumably later learnt of the death of his half-brother, Ari Haswari at Gibbs's hands when in reality, it had been NCIS/Mossad Liaison Officer Ziva David, who was Ari's paternal half-sister who had killed Ari with Gibbs taking the blame to shield Ziva.
Grief-stricken, Sergei at some point began monitoring the activities of various members of the NCIS Major Case Response Team as he planned his revenge against Gibbs.
NCIS Season 12
- Edgar Hussein - (shot twice in the chest).
- The helicopter attack - (shot down by a 9K-PSAM three casualties/fatalities):
- Lieutenant Joseph Hagen - (died on impact).
- Petty Officer Michael Evans - (died on impact).
- Lieutenant Commander Hannah Banks - (survived but died from her injuries hours after the crash).
- Leroy Jethro Gibbs - (survived the initial attack and later hunted; rescued).
- Timothy McGee - (survived the initial attack and later hunted; rescued).
- Kevin Hussein - (survived the initial attack and later hunted; rescued).
- The warehouse shooting - (all shot to death and moved the bodies to a diner; staged exactly how Jennifer Shepard was murdered):
- Master Chief Ekim Beanes - (stabbed once in the chest with a scalpel; staged exactly how Michael Franks was murdered).
- Diane Sterling - (shot once in the head; staged how Caitlin Todd was murdered).
- Global terror summit bombing (one casualty):
- Mikhail- (garroted by Renata Atal).
- Sofia Glazman- (garroted by Renata Atal).