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, Later Troi transferred to the USS Titan.

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, like 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 were limited compared to full-blooded Betazoids, and 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 and often came in handy when dealing with hostile races. Since she could usually tell if others were lying, she repeatedly proved herself invaluable in many suspenseful situations.

Deanna was born on March 29, 2336, near Lake El-Nar on Betazed, to Betazoid Ambassador Lwaxana Troi and Human Starfleet officer Ian Andrew Troi.

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.

As a child living on Betazed, Deanna often learned aspects of Human culture from her Human father, Ian. One such aspect which he passed on to his daughter was a fondness for stories set during Earth's Ancient West, which he often read to her. 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."

She entered Starfleet Academy in 2355. graduating in 2359, majoring in psychology. On her homeworld of Betazed, she met William T. Riker, a Starfleet ensign who was stationed there. The two began a relationship sometime between 2357 and 2361 which lasted several years. After Riker was assigned to the USS Potemkin the two planned to spend their holidays together on Risa in 2361. However, Will had to cancel their plans after he was promoted to lieutenant and the couple eventually lost contact to each other. By 2364, Troi held the rank of lieutenant commander and she was assigned as the Enterprise D ship's counselor.