The Emperor or Empress of the Romulan Star Empire was the (usually) ceremonial head of state of the Romulan Star Empire. The Emperor was formally ranked first in Romulan society, with his legate and the Praetor ranked behind him, but in most cases the Praetor is the true power in the Empire, with the Emperor and his legate fulfilling ceremonial roles. (TOS novel: Probe)
The first known Emperor of the Romulan Star Empire was Admiral Valkis, a collateral descendant of Tellus. This was during the initial formation of the Star Empire when they became an interstellar empire. After managing a victory over 'alien races', Valkis declared himself the Emperor with his highest Admirals and the leaders of the ancient families as members of the Imperial Romulan Senate. The Emperor would rule while the Senate managed the newly formed Romulan Star Empire. (Last Unicorn RPG module: The Way of D'era: The Romulan Star Empire)
In 1875, the Emperor was Vitellius, who was aboard a ship that encounted the planet Andor. By luck, the shields of the Emperors ship collapsed and the vessel was seriously damaged by the primitive Andorian weaponry. After crashing, the majority of the Romulan crew were killed leaving only a single survivor, Emperor Vitellius himself. He died shortly after and the Devil's Heart fell to the Andorian healer Telev. (TNG novel: The Devil's Heart)
After the Vulcan-Romulan War, a number of weak and decadent Emperors ruled the Empire which almost brought it to ruin until the reign of Emperor Valkis XIV who was deposed and replaced with a new figurehead who took on the title of Praetor that brought about a return to the ancient traditions of the Romulan people. (Last Unicorn RPG module: The Way of D'era: The Romulan Star Empire)
Following the Romulan Civil War of 2276, the formerly-exiled Ael i-Mhiessan t'Rllaillieu was proclaimed Empress. (TOS novel: The Empty Chair) By 2344, the aged Shiarkiek served as Emperor of the Romulan Star Empire. (ST novel: Vulcan's Heart) In 2356, an unnamed Empress held the title within the Star Empire and spearheaded an alliance with the Shepherd Lhaiu. (VOY novels: Cloak and Dagger, Shadow of Heaven)
In 2369, the current Empress, a fourteen year old girl, was infected with a lethal virus. She recovered after Dr. Leonard McCoy synthesized a cure from the uncontaminated blood of another member of the imperial line. (TNG novel: Red Sector)
- Given that Shiarkiek was Emperor for at least thirty years, during a period when there were at least two distinct Empresses, it would seem that either both titles are passed down in perpetuity through imperial lines, regardless of which one reigns, or that the female consort of an Emperor receives the title of "Empress" -- presumably the equivalent of a Queen consort rather than a Queen regnant.
The office would remain empty for decades, but in the years following the destruction of Romulus in the Hobus supernova Praetor Taris spoke of a possible renewed Romulan monarchy to unify the remnants of the Romulan people under one banner. In 2407 Taris' successor, Sela, revived the idea, and in 2408 declared herself Empress. Within a year, however, the office fell vacant again when Sela was kidnapped by an Iconian Quas-class starship, which tractored her flagship IRW Leahval during a battle in orbit of Brea III and dragged it through an Iconian gateway. (ST website: The Path to 2409; STO mission: "Cutting the Cord")