Deanna Troi was a half-Betazoid, half-Human Starfleet officer. Under the command of Captain Jean-Luc Picard, she served as ship's counselor aboard the USS Enterprise-D and the USS Enterprise-E. In 2379, Troi transferred to the USS Titan. (Star Trek: The Next Generation; Star Trek Nemesis)

As a half-Betazoid, Troi was capable of extra-sensory empathy, but was incapable of reading aliens with brain structures dissimilar to Humans and other Betazoids, such as the Breen and the Ferengi. Like most Betazoids, Troi had telepathic abilities. Due to her half-Human heritage, however, the range of her telepathic abilities was limited compared to full-blooded Betazoids, and she could usually only read the thoughts of other Betazoids, most notably her mother. Troi's empathic skills made her an important asset to the Enterprise-D and her abilities were often particularly useful when dealing with hostile races. Since she could usually determine, through use of her abilities, whether others were lying, she repeatedly proved herself invaluable in many suspenseful situations. (TNG: "Encounter at Farpoint", "Ménage à Troi", "The Loss"; VOY: "Inside Man")

Deanna was born on March 29, 2336, near Lake El'nar on Betazed. She was born to Betazoid Ambassador Lwaxana Troi and Human Starfleet officer Ian Andrew Troi. Deanna was actually the second of their two children, the first being Kestra Troi, born in 2330. However, Kestra drowned when Deanna was an infant. Stricken with grief and regret, Lwaxana removed all evidence of Kestra's existence from her own life and even repressed all memories of Kestra. She also made her husband swear never to mention Kestra again. Because of this, Deanna did not learn of her sister until 2370, when Lwaxana's memories resurfaced. (TNG: "Conundrum", "Dark Page")

As required by Betazoid custom, Deanna was genetically bonded with a Human male named Wyatt Miller at a young age. Wyatt was the son of Steven and Victoria Miller, close friends of Deanna's parents. Due to their genetic bonding, Deanna was scheduled to marry Wyatt in adulthood. (TNG: "Haven")

According to her mother, Deanna had a talent for languages as a child. (TNG: "Dark Page") During her adult life, Deanna also recalled having visited Enterprise NX-01 as a little girl, although she was not absolutely sure, as she always got "those museum ships" mixed up. (ENT: "These Are the Voyages...")

As a child living on Betazed, Deanna learned aspects of Human culture from her Human father, Ian. One such aspect was a fondness for stories set during Earth's Ancient West, which he often read to her. (TNG: "A Fistful of Datas") To fall asleep, she would ask him to sing the American song "Down in the Valley"; while this was her favorite, it was his singing that made her feel safe. (TNG: "Dark Page") As she was half-Human and half-Betazoid, she never felt trapped between the two worlds to which she belonged, instead choosing to embrace the richness of her dual heritage. (TNG: "The Emissary") In grammar school as a child, she had to memorize Jonathan Archer's speech at the Federation Founding Ceremony. (ENT: "These Are the Voyages...") She also heard stories from her maternal grandfather, who told them telepathically; something of a traditionalist, he rarely spoke, saying speech was for "offworlders and people who didn't know any better." (TNG: "Eye of the Beholder")