(JG)/Lieutenant/Lieutenant Commander Geordi La Forge was initially the ship's helmsman, but the character became chief engineer beginning in the second season. Burton also played the character in an episode of Voyager. Denise Crosby as Lieutenant Tasha Yar (season 1; guest: seasons 3 & 7) is the chief of security and tactical officer. Crosby left the series near the end of the first season, and the Yar character was killed. Yar returns in alternate timelines in the award-winning episode "Yesterday's Enterprise" and the series finale, "All Good Things...". Crosby also played Commander Sela, Yar's half-Romulan daughter. Michael Dorn as Lieutenant (JG)/Lieutenant Worf is a Klingon. Worf initially appears as a junior officer fulfilling several roles on the bridge. When Denise Crosby left near the end of the first season, the Worf character succeeded Lieutenant Yar as the ship's chief of security and tactical officer. Dorn reprised the role as a regular in seasons four through seven of Star Trek: Deep Space Nine and also played another Klingon, also named Worf, in Star Trek VI: The Undiscovered Country; with 282 on-screen appearances, Dorn has the most appearances of any actor in the Star Trek franchise. Gates McFadden as Doctor Beverly Crusher (Seasons 1 & 3–7) is the Enterprises chief medical officer. As a fully certified bridge officer, Dr. Crusher had the ability to command the Enterprise if circumstances required her to do so. She also, on occasion, commanded night-watch shifts on the ship's main bridge to stay on top of starship operations. McFadden was fired after the first season, but was rehired for the third season and remained for the remainder of the series. Her absence in the second season was explained by her transfer to Starfleet Medical. Diana Muldaur as Doctor Katherine Pulaski (Season 2) was created to replace Dr. Crusher for the show's second season. Muldaur, who previously appeared in two episodes of the original Star Trek, never received billing in the opening credits; instead, she was listed as a special guest star during the first act. Marina Sirtis as Lieutenant Commander/Commander Deanna Troi is the half-human, half-Betazoid ship's counselor. Starting in the season seven episode "Thine Own Self", Counselor Troi, having taken and completed the bridge-officer's test, is later promoted to the rank of commander, which allowed her to take command of the ship, and also perform bridge duties other than those of a ship's counselor. The character's relationship