Lieutenant Natasha "Tasha" Yar was the chief security officer of the USS Enterprise until her death in 2364.
Tasha Yar was born in 2337 on the failed colony planet Turkana IV. Growing up as an orphan in a lawless wasteland, she struggled to raise her younger sister. At the age of fifteen, she managed to leave the planet and joined Starfleet.
Yar graduated Starfleet Academy and became a security officer, inspired by her childhood experiences. At some point before 2363, she was involved in a rescue mission where her actions were noticed by Captain Picard. He arranged for her to be transferred to his command, eventually bringing her along when he took command of the Enterprise-D.
Her career aboard the Enterprise, however, was brief.
Yar was killed during the recovery of a crashed shuttle. When the rescue party was blocked from reaching the crash survivors by the being known as Armus, Yar attempted to bypass the creature; it then attacked and killed her as a display of its power.
Tasha Yar (2344)
Yar reappeared twenty years earlier at the Battle of Narendra III, serving as tactical officer aboard the Enterprise-C. This vessel had been temporarily brought to an alternate future where Yar had not been killed by Armus and continued to serve as the Enterprise-D's head of security. Upon learning that her death was inevitable in either universe - through Armus or through the disastrous Klingon War - she chose to join the surviving crew of the Enterprise-C in their certain defeat.
Yar survived the battle and was taken to Romulus as a prisoner. She married a Romulan officer in exchange for the surviving crewmembers' lives and gave birth to a daughter, Sela. Yar was once again killed in 2349, when a crying Sela gave away her escape attempt.
In the non-canon novel Star Trek Titan: Over A Torrent Sea, Deanna Troi and William Riker name their daughter after Lieutenant Yar