Ishara Yar was a Human female, 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 who later abandoned them, leaving Tasha to care for them both.
In 2352, when she was old enough, Ishara allied herself with the Coalition. Her sister left the planet as soon as she was able but Ishara refused to join her – she had already joined the Coalition and considered leaving cowardly.
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.
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 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. 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.
However, while 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, then ultimately sent her back to her cadre, Captain Picard reflecting that they had all wanted to see more of Tasha in Ishara than had apparently been there. 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")
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.