This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
Tallinn was a female Romulan supervisor who operated on Earth during the 21st century. Her assignment was to monitor and safeguard Renée Picard, and to ensure the proper unfolding of Earth's history in doing so. As a supervisor, she was furnished with various advanced tools, including a servo, a scanner, a transporter, and a neuro-optic interceptor that allowed her to take control of other people, using the bodies of these "hosts" to converse remotely. She also used technology to appear Human, which if switched off could not be used again for eight hours. (PIC: "Watcher", "Two of One", "Monsters")
Tallinn ultimately sacrificed herself to ensure Renée Picard's survival, first introducing herself to the young woman and explaining her role in Renée's life. Q later told Jean-Luc Picard that Tallinn was destined to die in every timeline, but Picard's intervention meant that she at least got to meet Renée in person in this one. (PIC: "Farewell")