Reman

Remans are the humanoid inhabitants of the planet Remus. They have been the slave labor caste of the Romulan people since at least the 22nd century and have been forced to work in the extremely hazardous dilithium mines of their world. (ENT: "United", "The Aenar")

As Remus is a tidally locked planet, the Remans live on its dark side, and thus are extremely sensitive to light. Some Remans, if not all, posses telepathic abilities similar to those shown by Vulcans and Betazoids. (Star Trek: Nemesis)

Among the Romulan people Remans have historically been known as great warriors, as such Romulan senators often had Reman bodyguards in thier employ to intimidate possible opponents. (ENT: "United", "The Aenar")

Remans were used by the Romulan Empire as "cannon fodder" during the Dominion War, and were often deployed in the fiercest engagements.

In 2379, the Remans usurped the power of their Romulan overlords in a military coup, installing thier leader, Shinzon (in actuality a human), as the new Praetor of the Romulan Senate and thus the empire. As an act of war against the Federation, Praetor Shinzon, with his warbird the Scimitar, then attempted to destroy Earth. However, with the destruction of the Scimitar at the Battle of the Bassen Rift, after Shinzon's supporters on Romulus turned against him, the bid for Reman superiority prematurely ended. (Star Trek: Nemesis)

Mirror universe
In the mirror universe, the Remans were genetically engineered Romulan offshoots who served as the military arm of the Romulan Technocracy. A former praetor resided aboard the Reman warbird Sarthax in 2385. They seemed to coexist peacefully with their Romulan brethren. 

Background

 * Canonically, it is not known whether the Remans are descended from Romulans who colonized Remus or are native to the planet. It's possible that Remans are the product of interbreeding between Romulans and some species native to either Romulus or Remus, or they could be genetically-modified race with Romulan DNA, or similar to human Augments. The somewhat similar Rigelian race (who share blood chemistry with Vulcans) seems to support the notion that the Remans are similar to Vulcans as well.
 * Reman telepathy may be further evidence of at least partial Vulcan ancestry. The non-canonical USS Titan novels suggest that Remans are, in fact, Romulan offshoots.
 * It has been speculated that the Remans fought in the Earth-Romulan War in place of Romulan soldiers. This would seem to explain why Romulans and humans never came face to face in that war, but it is merely speculation.
 * In the last moments of the Star Trek: Enterprise third season cliffhanger "Zero Hour", an alien appears in a Nazi uniform. Many fans speculated that the alien was a Reman; however he was later revealed to be Na'kuhl, a species that did share a certain similarity with the Remans.
 * While they didn't appear as Nazis, Remans appeared in the Enterprise season four episodes "United" and "The Aenar", albeit briefly.
 * Many fans have wondered how the Remans were able to build large vessels such as the Scimitar in secret. Dialogue in Star Trek: Nemesis indicated that they also had a secret base of operations, but this doesn't explain how such an oppressed people developed such a well-equipped army.

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