Rita was a Dauntless member. After the Choosing Ceremony, she was seen crying on the roof, being held back by a Dauntless boy ( supposably her boyfriend) to prevent her from falling off, where the Dauntless initiates jumped unto from the train. On the ground was her sister's dead body that fell during their jump showing Tris that even the Dauntless weren't immune from harm, from a mistake or from a slip up.
She is often remembered by Tris when she is climbing, remembering Rita's sobs and her sister, bent at weird angles.
In Allegiant she is mentioned in Tris's POV. She was the last jumper