Dr. Beverly Crusher (Gates McFadden) served as the Chief Medical Officer on the USS Enterprise-D for six out of Star Trek: The Next Generation‘s seven seasons, but her Starfleet rank was rarely mentioned. Introduced in TNG’s first episode, Gates McFadden was a Star Trek: The Next Generation series regular. Dr. Crusher was one of the original crew members of the Enterprise-D, serving under the command of Captain Jean-Luc Picard (Patrick Stewart). Crusher and Picard had romantic feelings for one another, although they didn’t officially embark on a romance on screen. Crusher lived on the Enterprise with her child-prodigy teenage son, Wesley Crusher (Wil Wheaton).

https://screenrant.com/star-trek-doctor-crusher-rank-tng-explained/