What's the history between Naomi Nagata and Marco Inaros in Rocinante crew and one of The Expanse's core protagonists, Naomi is a Belter carving out a living on a ice hauler when the story begins. One of only five survivors from the fateful Canterbury attack, Naomi's shady past comes into view during her interrogation with the Martians but, despite keeping more than a few skeletons in her closest, Naomi proves herself a friend to people from all walks of life, and a key figure in mankind's fight against the Protomolecule.
The Expanse season 5 is Naomi's most personal story yet. Previous seasons alluded to her younger years and sporadically revealed slithers of information, but The Expanse has now reunited Naomi with Marco Inaros, her former lover turned terrorist, and Filip, their grown-up son. Naturally, the family get-together brings the ghosts of Naomi's past to the fore, and reignites the unresolved tensions between them. But what exactly went down between Naomi and Marco in the time period before The Expanse began?
Born on Ceres Station in the Belt, Naomi faced adversity from the very beginning, but strove for a better life where she could use her natural affinity for engineering to aid the ongoing Belter cause. It was during these formative teenage years of education that she met and fell in love with fellow Belter, Marco Inaros, then part of a salvage crew. The couple rode with Cyn and Karal (both still allies of Inaros in The Expanse season 5) and became a family unit of sorts, especially after Naomi gave birth to Filip. Despite the "alliance" tag of the OPA, vary wildly in their methods and philosophies. While Naomi was relatively moderate in her outlook, Marco believed in punishing the Inner planets with acts of violence, and this difference of opinion was ultimately what drove a final wedge between them.
While travelling together, Naomi used her engineering smarts to develop a computer virus that could shut down a ship's reactor, causing trouble for Earth and Martian vessels. But Marco wanted to do more than merely "cause trouble," and he repurposed Naomi's code to overload a reactor instead of shutting it off. The first example of this weapon in action was the civilian Augustin Gamarra vessel. Naomi was understandably devastated that her code had been turned into a weapon of mass destruction behind her back - and by her own lover, no less. She promptly left Marco Inaros behind and struck out for a new life away from the OPA and her former friends, eventually falling into the Rocinante crew. However, when Naomi left Marco, she also walked away from their son, who Marco refused to relinquish. Since Naomi left roughly 16 years prior to The Expanse, Marco has risen to prominence as an OPA revolutionary, and told anyone who'll listen that Filip's mother is a traitor to the Belt.
Naomi's past with Marco explains how she realizes the Rocinante is in danger in The Expanse season 5's latest episode. Filip tells his mother that he saved her life by taking her to Marco's ship, and Naomi obviously knows about the dangerous code Marco possesses. While she hasn't yet learned that Sakai was secretly working for the Free Navy, Naomi hears enough to realize that Marco could've feasibly slipped the Augustin Gamarra virus onto the Rocinante during the repairs at Tycho station, and makes the call to Holden just in time to prevent a deadly launch.