Tara Mishra

 was a companion of the Ninth Doctor. She was born in Sri Lanka. (PROSE: Christmas Special)

Before joining UNIT, Tara was a nurse.

She met the Ninth Doctor, Rose and Jack in Blaise Castle. She later revealed herself to be a member of UNIT.

Tara later sneaked aboard the Doctor's TARDIS, and greeted them. She then joined them on their next adventure. (COMIC: Official Secrets)

The Doctor concocted a plan to have himself arrested for Tara's murder. Tara herself was alive and well. Her part of the plan was to free the Doctor from his imprisonment in the Hesguard Institute. (COMIC: Sin-Eaters)

After a few adventures in the TARDIS, the crew were brought off course to the site of a chat show hosted by Slist Fayflut Marteveerthon Slitheen. (PROSE: Christmas Special)

Legacy
While recounting an event in the Ninth Doctor's life to Bill Potts, the Twelfth Doctor clarified that it was "before Tara". (COMIC: The Promise)