During the 1980s, the Romulans took center-stage in “Rihannsu,” a five-novel series written primarily by Diane Duane (Peter Morwood co-authored the second, “The Romulan Way.”) Published from 1984 to 2006, the novels invented a culture and language for the Romulans wholesale; they are technically not “Trek” canon but remain acclaimed for their world-building.
“Rihannsu” (“The Declared”) is the Romulans’ native name for themselves, akin to how German people call their nation “Deutschland,” the Japanese call theirs “Nippon,” etc. They are driven by “D’era,” an expansionist impulse akin to Manifest Destiny, and “Mnhei’sahe” (ruling passion), a complex code of conduct that is foremost a rejection of the Vulcan system of logic.
“Mnhei’sahe” is weighed by one’s personal strength and devotion to the Empire. Romulans seek power not for personal benefit per se, but because greater power serves the Empire. Selflessness is an alien concept to the Romulans; do things for the sake of your own Mnhei’sahe and others will benefit in the process. “Mnhei’sahe” spreads its claws even into simple Romulan social interactions, where the ideal outcome is for both parties to depart with their honor intact.
Much of Duane’s other additions are inferences based on “Balance of Terror” and “The Enterprise Incident.” Akin to Rome, the Romulans are an Oligarchic Republic; a Praetor is elected by the Senate itself, not the people at large. “The Enterprise Incident” showed a Romulan woman with a high military rank. So, “Rihannsu” gave the Romulan society a matriarchial tilt; a Romulan’s family lineage is derived from their mother, not their father.
Much of “Rihannsu” is a holdover from the suggestion in “Star Trek: The Original Series” that Romulans were a warrior culture. For instance, their society has a semi-feudal system with a strong emphasis on family affiliation. Canon material would take a different path, showing Romulans as militant but not exactly honorable.