Sarina's parents took her as a child to undergo an illegal procedure called accelerated critical neural pathway formation which dramatically increased her intelligence beyond what Humans would call genius level. Unfortunately, though her cerebral cortex was enhanced to process information faster, it couldn't initially get that information from her visual and auditory systems fast enough, and Sarina was unable to focus on her world. Outwardly, this made her appear mute and oblivious to her surroundings.
In 2360, Douglas was introduced to other genetically-engineered Humans Jack, Patrick and Lauren at a Federation Institute for special needs individuals unable to function in normal society. She would spend fifteen years with them. During this time, a Doctor Karen Loews was assigned to her and the others in the group. She separated Douglas and her group from the others in order to treat them differently. Loews also became Douglas' legal guardian. (DS9: "Statistical Probabilities", "Chrysalis")
In 2374, Douglas and the others were brought by Loews to Deep Space 9 to be placed under the study of Doctor Julian Bashir, who himself was genetically-enhanced, for three weeks. At first, the savants proved to be helpful to the Federation. They pointed out that Weyoun was willing to cede extra territory in order to gain a planet where ketracel-white for the Jem'Hadar could be produced. As Sarina was mute at the time, she managed to convey said information to Doctor Bashir by sketching a chemical diagram on a PADD.
Later on, however, the savants calculated that the Federation could not defeat the Dominion and should surrender before there was an appalling loss of life. When Captain Sisko refused to consider their proposal, the savants decided the best way to prevent mounting casualties was to contact the Dominion and give them information that would allow them to take over the Alpha Quadrant with little loss of life. When Bashir tried to stop them, they rendered him unconscious, tied him up, and left Sarina to guard him. But Bashir was able to get through to Sarina and convince her to release him, and he was able to stop the savants from carrying out their plan. The savants left Deep Space 9 soon after. (DS9: "Statistical Probabilities")
In 2375, Jack got word that Bashir wanted to try to treat Douglas using a procedure to allow her brain to gather information from her senses, which would allow her to live a normal life. He led the group to bring her to Bashir on Deep Space 9, posing as officers. Using a neurocortical probe, refined by Jack and Patrick, Bashir was successful in stimulating the growth of the synapses in her brain, allowing her to interact with people for the first time in her life.
Douglas was beyond thankful for Bashir's actions, and took delight in interacting with her surroundings. With the help of her friends, she was able to quickly re-learn how to speak, and sing. She had a bit of awkwardness fitting back in with her friends in subsequent conversations, but they were still family to her. She helped Bashir with a research problem he'd been working on, and played dabo with her skills in probability theory. She and Bashir became very close, and, indeed, fell in love. The quickness of it however nearly drove her back into seclusion when she didn't know how to respond to his advances. Ultimately, she left Deep Space 9 to pursue an internship Bashir arranged for her at the Corgal Research Center. (DS9: "Chrysalis")
"What is he talking to her for? He read the reports. She won't answer. Didn't you read the reports?"
"Hello, Sarina. Remember me?"
"Of course she remembers you. She's not an idiot."
"She's just a little cataleptic."
"Are you really going to be able to make her better?"
"I'm going to do everything I can. I promise."
"Sarina? What are you looking at?"
"Everything. You heard me. I thought something and you heard it."
"Sarina, you spoke."
"I was listening when you were telling the nurses about the procedure. I kept thinking that I wanted to thank you for what you were trying to do for me, and now I finally can. Thank you."
"Did you hear what she said? Did you hear that?"
"I don't think I've heard a more beautiful sound in my life."
Sarina was played by actress Faith C. Salie.
Two of the costumes worn by Salie in her appearances as Sarina were later sold off in the It's A Wrap! sale and auction. The first, a short-sleeved dress featuring multi-colored, thin, vertical stripes was sold for US$168.50,  while her second costume, a long-sleeved shirt in a light green and black with matching trousers and light green socks, was sold for US$167.50. 
The characters of Jack, Patrick, Lauren, and Sarina were collectively referred to as the "Jack-pack" by the Deep Space Nine writing team, with each character having a specific personality trait. Sarina was described as the "astrophysicist who couldn't talk" and was a holdover from an original story idea for "Statistical Probabilities" called "Think Tank" and involved Starfleet sending the group, including Sarina, to Deep Space 9 as a think tank on the Dominion War. (Star Trek: Deep Space Nine Companion, p. 513)
Originally, Sarina was to have spoken in "Statistical Probabilities", but the lines spoken by actress Salie ultimately ended being cut from the finished episode. "When I auditioned for the part, I had four lines of dialogue in the pivotal scene where I decide to untie Bashir." Salie recalled. "And, in fact, that's the way we filmed it. So it was much to my surprise when I sat down with my family at Thanksgiving to watch it and discovered I'd turned into a mute." As René Echevarria explained, "The episode was long, so we were looking for cuts. And it just played better when Bashir made his case to this woman and you didn't know what she would do until the next scene, when you discovered Bashir had been freed. So we cut Sarina's dialogue in the editing room. I remember thinking, 'Oh God, this poor woman. She's probably told everybody she knows about this scene.'" (Star Trek: Deep Space Nine Companion, p. 514)
One of the reasons the cut worked well was because Salie had been directed to behave as if Sarina couldn't speak. "I was told to behave pretty much catatonic," she recalled. "Anson [Williams] told me 'There's a lot going on in your mind, because you're genetically enhanced and you're brilliant, but you can't facilitate it because your body doesn't know how.' I don't want to make it seem like some terribly difficult actor's moment, but I did work on it. I created a switch in my brain that I could turn on and off to make everything become hazy around me, so that it seemed as if an amalgam of voices and senses were coming at me and that it was overwhelming. Of course, sometimes it was as simple as trying not to blink! In most of my scenes, I kept myself busy by focusing on the wall and touching it. Michael Keenan kept making jokes about me playing the wall. He said, 'I'm so glad those wall lessons your parents gave you paid off.'" (Star Trek: Deep Space Nine Companion, p. 514)
As it turned out, the idea not to let Sarina speak in the episode, provided the inspiration for her next appearance in "Chrysalis". However, because the role was to become dialogue heavy, the producers were concerned that Salie might not be up to it. "I had to go through a few hoops." Salie remembered. "Before offering it to me, they auditioned me. But to be fair to them, they didn't call in anyone else." Echevarria commented, "We certainly didn't want to recast, but we did bring Faith in to read. It must have been incredibly stressful for her. She had a role that was hers to lose. But within seconds of her reading, we knew she was in." Similarly, Ira Steven Behr said, "We suspected that Faith would be fine. I'd seen her do some children's theater, and I knew she was a very intelligent woman." (Star Trek: Deep Space Nine Companion, p. 614)
Although the character had appeared before, the writers had never developed a specific personality for her. "They explained to me that she should have no subtext," Salie explained. "which is a very unusual feature for a character. As an actor, you're taught to dig, dig, dig to find out what's underneath the lines and to know the backstory, because often a character might be saying something that she really doesn't mean. But with Sarina, there was none of that. She was guileless. She hadn't been around enough to learn about disingenuousness or even flitation. The undercurrent in almost every scene in film or television between a man and a woman is one of some sort of flirtation or sexual attraction. But I was told specifically to take that out of my scenes with Bashir. In no way was I supposed to relate to him flirtatiously, It sounds like a simple task - 'Just say the line and mean it' - but that was hard to play, because we're never trained to take lines at face value." (Star Trek: Deep Space Nine Companion, p. 614)
The scene in which Sarina is introduced to the station's Promenade was much easier for Salie. "The word used in the script's stage direction was 'astonished'. Well, the way the soundstage was set up, that wasn't hard to play! The Promenade set just bombards you with colors and flags, and I was surrounded by aliens. And, it's always very exciting to have the camera right in your face. I'm not over that yet." Director Jonathan West helped Salie for the scene by comparing the moment to that of a well-known film. "Jonathan called that shot the 'Maria moment'." Salie remembered. "The camera was going to zoom down on my face when my mouth was wide open. Jonathan said, 'Just think about Maria on top of the mountain in The Sound of Music, twirling around and around." (Star Trek: Deep Space Nine Companion, p. 614)
The musical sequence in "Chrysalis" was developed by Rene Echevarria when it struck him that Sarina might have difficulties vocalizing. "That's why I thought they should start singing," he recalled. "Now the scene establishes the vocal problem, gets away from it, and shows everybody's joy. And most importantly, we fall in love with Sarina and we certainly see Julian falling in love with her. I was just delighted with the scene in the script but Ira [Steven Behr] was skeptical." Initially, the producers were worried about Salie's ability to pull the singing off, but she turned out to be the most proficient vocalist. "As soon as I read about the singing in the script I was so excited." Salie enthused. "Because I'm musically trained. I don't think any of the other actors enjoyed doing it as much as I did." (Star Trek: Deep Space Nine Companion, pp. 614-615)
Salie commented: "the most iconic show I've ever been on was Star Trek: Deep Space Nine. I still get fan mail. I'm on a trading card. My character lives on in a DS9 novel". 
In his review of "Chrysalis", Star Trek author Keith R.A. DeCandido wrote, " It's actually fun to see the Jack Pack again, all four actors do superbly, especially Faith Salie, who had a much bigger role to play this time as Sarina. Salie is radiant and wonderful, and really sells Sarina's transition."  Similarly, authors Mark Jones and Lance Parkin wrote, "Salie turns a pretty unpromising story into something with a little more depth." (Beyond the Final Frontier, p. 259)
The final script for "Statistical Probabilities" gives the pronunciation of Sarina's name as "suh-REE-nuh" and goes on to describe her as being as "white as a ghost." 
Sarina Douglas is referenced in a number of non-canon works.
In the Prophecy and Change short story "The Devil You Know", when Jadzia Dax informs Julian Bashir of her research into modifying the Jem'Hadar, Bashir tells her for every Julian Bashir there is a Jack, a Patrick, a Sarina and a Khan Singh.
In the Section 31 novel Abyss, it is mentioned that Cole is aware of Douglas, and Bashir's involvement with her. Disavowed reveals Sarina moves to Andoria to live with Bashir, accepting a position with the Federation Security Agency, however this is a cover for her actual position as a member of Starfleet Intelligence. She and other members of Starfleet intelligence work to insert Bashir into Section 31 while the organization is still interested in him. Cole recruits Bashir and Douglas for a mission to the mirror universe.
In the Typhon Pact novel Zero Sum Game, at some point prior to 2382, Sarina is contacted by Section 31 operative L'Haan and recruited into the agency as an operative. During August of 2382, Sarina and Julian Bashir are assigned a mission to Salavat to investigate intelligence reports suggesting that the Typhon Pact is constructing a Quantum slipstream drive-powered vessel at a facility in that system. During the mission, Sarina is captured and interrogated by the Breen, but is able to escape and make the rendevous with the USS Aventine. During the mission Sarina and Bashir reunite and start a romantic relationship. Nevertheless, it is later revealed that Sarina has accepted an assignment within Section 31 and is tasked with recruiting Bashir into the agency by her handler, L'Haan.
Following this, as told in Plagues of Night and Raise the Dawn, Douglas later requests a transfer to Deep Space 9 and is assigned to the security team under Jefferson Blackmer, holding rank of lieutenant. After surviving the destruction of the station, she is promoted to lieutenant commander and assigned first to Bajoran Space Central and later to the new station. Sarina reveals to Ro Laren that she is working against Section 31 from within.
Bashir and Sarina continue their quest to destroy Section 31. Ultimately, in Control, they discover that Section 31 is being run by an artificial intelligence called Uraei, also known as "Control", which had its origins in the 22nd century. With Data's help, a computer virus is created to eliminate Uraei, but Sarina is brainwashed by Section 31 and made to fight Bashir as he attempts to install the virus. Bashir succeeds, but is severely wounded, and Sarina is made to commit suicide by Control. Section 31 is exposed to the public and dismantled but unknown to anyone, the Control persona survives independently of Uraei and goes into hiding.
An alternate Sarina appears in the Myriad Universes short story "Seeds of Dissent". In that timeline, Khan Noonien Singh has won the Eugenics Wars and Humanity underwent genetic engineering en masse. Doctor Sarina Douglas is an Augment serving aboard the Earthfleet warship Defiance, under the command of Princeps Bashir, in 2376. When Constantin Amoros leaves the ship as part of the boarding party to the SS Botany Bay, Douglas is the senior medical officer on board. She informs Bashir of the conditions of those who are in suspension aboard the Botany Bay and that they are "Basics" - Humans without genetic enhancements.
Sarina is also featured in the Star Trek Customizable Card Game which describes her as a "Cataleptic Conundrum".