In 'Star Trek', the Romulans are related to what other aliens?

The Romulans are an extraterrestrial humanoid species in the science fiction franchise Star Trek. First appearing in the original Star Trek series in the 1966 episode "Balance of Terror", they have since made appearances in all the main later Star Trek series: The Animated Series, The Next Generation, Deep Space Nine, Voyager and Enterprise. In addition, they have appeared in various spin-off media, and prominently in the two films Star Trek: Nemesis (2002) and Star Trek (2009).

The Romulans also act as a counterpoint to the logical Vulcan race, whom they resemble and with whom they share a common ancestry. As such, the Romulans are characterized as passionate, cunning, and opportunistic — in every way the opposite of the logical and "cold" Vulcans. The Romulans are the dominant race of the Romulan Star Empire. Although Star Trek Star Charts place the Romulan Empire's territory in the Beta Quadrant of the galaxy in Star Trek: Deep Space Nine, they are referred to as an Alpha Quadrant power.

Throughout the series, they are generally depicted as antagonists, and are usually at war with or in a tenuous truce with the United Federation of Planets. On extremely rare occasions, they have allied with the Federation. They generally do not get along with Klingons either, whom they consider to be a savage race, while the Klingons consider Romulans dishonorable.