Commander Arla Rees was a 24th century Bajoran Starfleet officer. In 2374, she was considered by Benjamin Sisko to be exceptionally intelligent, attractive, young, and very serious, with sharp edges that had yet to be blunted by the realities of routine. Arla had almond-brown eyes and olive-gold skin.
Arla's grandparents lived on B'hal Ta, a Bajoran colony world. When the Cardassians annexed Bajor, her family relocated to New Sydney, where she was born. Like many expatriate Bajorans, Arla was considered by many Occupation survivors to be only one step removed from being a collaborator. Unlike many Bajorans, Arla was not a believer in the Prophets, and in fact believed that the Bajorans' "delusional worship of the Prophets turned them into the galaxy's biggest victims."
While at Starfleet Academy, Arla became acquainted with Boothby. Arla also became a by-the-book Starfleet officer, with a firm belief in the infallibility of Starfleet regulations. On the Day of Withdrawal, Arla was serving aboard the Vulcan science vessel, Solok, remapping the Praxis Ring near the Klingon homeworld Qo'noS.
During the USS Defiant's mission to escort the PQ-1 convoy to the Vegan system on stardate 51948.3, Arla was unexpectedly assigned as second-in-command of Deep Space 9 to coordinate with Major Kira Nerys. Despite Kira's initial fears that Arla's presence was a gesture of no-faith by Starfleet, it became clear that Arla's assignment was temporary. She was slated for the number-two position on Farpoint Starbase's command staff, and Starfleet felt she could benefit from experiencing life on Deep Space 9, a "living laboratory of cross-cultural complexity."
One of her initial assignments on Deep Space 9 was to report on Starfleet-Bajoran cultural referents in the workplace. She delivered a 30-minute presentation on her time-and-motion modification studies that arose from observations of Bajoran and Starfleet personnel working together, which she accompanied with unrealistic suggestions to improve efficiency that were rejected by Captain Sisko, including the banning of group religious celebrations during wartime.
Arla's relations with members of the command staff were uncomfortable at best. She was rebuffed after subtly coming on to Captain Sisko. She was barely tolerated by Major Kira due to her absence from Bajor during the occupation. Arla also made known her opposition to Bajoran religion, engaging both Captain Sisko and Chief O'Brien in awkward conversations on the topic.
Soon after the murder of Andorian scientist Dal Nortron, Arla staffed the flight operations console of the USS Defiant and actually crewed the ship single-handedly, a prospect she seemed incapable of coping with, especially since she hadn't piloted anything larger than a shuttle since she'd graduated the Academy. She also held this position during the Defiant's subsequent accidental trip to the year 2400. (DS9 - Millennium novel: The Fall of Terok Nor)
While the Defiant was trapped in the combined Red and Blue Bajoran Wormholes, Arla had a vision that she was trapped in a Pah-wraith hell where the Prophets had forsaken her because she had forsaken them. When she would ask for their help they would proceed to leave to allow her to be raped and murdered by Cardassian soldiers during the Occupation of Bajor. This would have a profound effect on her and would help drive her deeper away from the Prophets.
After awaking from her Pah Wraith hell, she was assigned to keep watch over the unconscious Kai Weyoun in the Defiant Sickbay while the Senior Staff of the vessel worked on a plan of action. When Weyoun awoke he offered to share with her the secrets of the Grigari which she accepted. She helped Weyoun to escape the ship and then faked injury to make it appear as if she were forced. After she awoke she revealed to Jake Sisko that she was a follower of the True Prophets of Weyoun and attacked Commander Worf, severely injuring him. She later murdered two technicians aboard the ship (one who was treating an injury inflicted by Worf and the other by activating the shields of the Defiant while he was beaming off the ship). She proceeded to play a game of cat and mouse with Jake Sisko who had barricaded himself in the Defiant's Engine Room. When the battle between the Prophets and the Pah'Wraith ended she was returned to the normal timeline. She joined Prylar Obanak Keelan as a guardian of the Red Orbs of Jalbador. (DS9 - Millennium novel: Inferno)