Yefti is a Na'vi from the Sarentu Clan who was one of the participants in the Ambassador Program. Yefti, like the other TAP children, was kidnapped from their clan by John Mercer when they were a small child and forced to participate in the program.
In 2154, after a forced evacuation due to Jake Sully's rebellion, Mercer ordered the children killed. They were saved by Alma Cortez, who personally killed the soldiers sent by Mercer and then put the children into cryosleep.
Ri'nela mentions Yefti at the old TAP school. She wonders where they and Telisi have gone.