Commander Deanna Troi /diːˈænə/ is a main character in the science-fiction television series Star Trek: The Next Generation and related TV series and films, portrayed by actress Marina Sirtis. Troi is half-human, half-Betazoid and has the psionic ability to sense emotions. She serves as the ship's counselor on USS Enterprise-D. Throughout most of the series, she holds the rank of lieutenant commander. In the seventh season, however, Troi takes the bridge officer's examination and is promoted to the rank of commander, but continues as counselor. Deanna and Riker are the last Star Trek: The Next Generation main cast members to appear on television, in the 2005 finale of the later Star Trek: Enterprise TV show. Deanna appears in all four TNG theatrical films, and also in three episodes of Star Trek: Voyager.
Her romantic interests, family, and personal life are plot elements in many Star Trek: The Next Generation episodes over the course of the series.