Dr. Charles Edward Eppes is portrayed as a young mathematical genius and professor of applied mathematics at the fictional California Institute of Science, CalSci (primarily based on Caltech, where some filming and mathematics consulting is done). As a world-class mathematician, Charlie helps his brother Don Eppes solve many of his perplexing FBI cases, sometimes with the help of his best friend, mentor and colleague Larry Fleinhardt and his on-again off-again girlfriend, former student and now wife, Amita Ramanujan, who further refines Charlie's approach and helps him stay focused. Charlie has consulted for the National Security Agency (NSA), in part as a cryptanalyst, for nearly five years, having attained TS/SCI security clearance. It was revoked at the end of season four after he transmitted information to Pakistan, but has recently been re-instated.