Yar was born on Turkana IV in 2342. Ishara's parents were killed just after she was born, after which she and her elder sister, Natasha, were looked after by "some people" for a few months before they inexplicably disappeared, leaving Tasha to care for them both.
In 2352, when she was old enough, Ishara allied herself with the Coalition and was subsequently implanted with a proximity detector. Her sister soon left the planet when she was able. She asked Ishara to come with her, but she refused – she had already joined the Coalition and considered them her family. She would always consider Natasha a coward for leaving, and would never see her again. Over the next several years Ishara would gain extensive knowledge about the tunnel system in the colony.
In 2367, the USS Enterprise-D came to Turkana IV to rescue crew members who had escaped from the Arcos after a warp core breach. The Turkana IV Alliance had captured the two crew members and were holding them hostage in exchange for weapons. Ishara volunteered to help the Enterprise-D in its rescue mission as part of a Coalition plot. She supplied the Enterprise-D with the location of the crew's escape pod, maps of the Alliance tunnels, and ultimately volunteered to create a diversion by setting off the Alliance proximity detectors so that Geordi La Forge could track the missing crewmen. She was stunned by a phaser blast, but Commander Riker brought her back safely to the Enterprise-D.
Ishara also appeared to grow close to the Enterprise-D crew (Lieutenant Commander Data in particular) and indicated she wanted to leave the Coalition and perhaps one day join Starfleet. She also asked about Natasha and, after hearing about her, appeared to change her mind about her. Doctor Crusher removed the implant that identified her with the cadre and she agreed to accompany the away team on a second mission to recover the Arcos crew. This time, she would not set off alarms.
However, once in Alliance territory – deeper than she could ever have gotten with the implant – Ishara took the opportunity to sabotage the fusion reactor that powered the Alliance defensive systems in order to enable a massive invasion by the Coalition. Data and Riker intervened by stunning Ishara so she could not shut down the reactor. Captain Picard, reflecting that they had all wanted to see more of Tasha in Ishara than had apparently been there, ultimately sent her back to her cadre instead of holding her for endangering Starfleet officers. As she left, however, she told Data that the time they had spent together was the closest thing to friendship that she had ever known. (TNG: "Legacy")
Whether intentional or not, Ishara's name closely resembles the Hebrew words "Isha ra'a" (אישה רעה), meaning "bad woman."
Although she has not appeared in other media, in the novel Q-Squared, when the actions of Trelane cause three alternate realities to collapse into each other, Worf convinces an alternate Tasha Yar to help him escape Enterprise crew members from a third timeline where the Federation is at war with the Klingons by telling the alternate Tasha that Ishara regretted all the unhappiness between them, demonstrating a personal knowledge of her that convinces the alternate Tasha to trust him.