Sarina Douglas was a genetically-engineered Human. 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.
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, when Sarina and the others returned to the station, Bashir devised a procedure to allow her brain to gather information from her senses, which would allow her to live a normal life. Using a neurocortical probe refined by Sarina's genetically-enhanced friends, Jack and Patrick, Bashir stimulated the growth of the synapses in her brain, allowing her to interact with people for the first time in her life.
After seeing the procedure's success and getting to know Sarina, Bashir fell in love with her, nearly driving 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 at the Corgal research center.
In the novel Zero Sum Game, Sarina had become an agent of Starfleet Intelligence. She accompanied Bashir on a mission to stop the newly established Typhon Pact from obtaining slipstream propulsion. During that mission, Sarina reunited with Bashir, starting a longer-standing romantic relationship. The depth of her feelings for him was unclear since Sarina was entangled in Section 31's latest attempt to recruit Bashir.