Reni is the enemy girl who was Lee's girlfriend for part of their stay in Stratton. They met when Lee found her naked and struggling to swim. She evidently sold him out after several dates, and had planned for him to be killed, but with the help of Ellie, he got away.
Ellie called her a slut and made a comment like "Lee nearly got us all killed for sex." When talking about how it affected the group.