FAQ for
Star Trek: Nemesis (2002) More at IMDbPro »

The Remans are the native inhabitants of the planet named Remus, which is the sister planet of planet Romulus. Romulus and Remus were named as such by the Romulans, who were originally a group of Vulcans who had left Vulcan two millenia earlier, before the Vulcans became an enlightened species who embraced logic. The Romulans have therefore retained much of their former brutality. When settling on Romulus, they also annexed the neighbouring planet, dubbed it Remus and subdued the native people, who were henceforth called 'Remans'. The Remans are virtually slaves, performing forced industrial labour on Remus and serving as expendable soldiers in the military, without the possibility to rise up to command or political functions.

Being a completely different race explains the vastly different appearance of Remans compared to the Romulans. The Remans have only ever inhabited the dark-sided hemisphere of the planet, causing their pale appearance and sensitivity to light.