Cameron Dennis is an attorney and was Harvey's mentor, along with Jessica. He was the Manhattan District Attorney who Harvey worked under as Assistant District Attorney before working at Pearson Hardman. He has questionable ethics and is willing to engage in unlawful or illegal acts in order to win cases.
In Rules of the Game, Cameron is accused of and investigated for burying evidence, and Harvey is forced to testify against Cameron. Being his mentor, Harvey is unwilling to help form a case against Cameron, so Harvey asks him to just step down from his position and make a deal with the other side. When Cameron realizes that Harvey's honor might just make him talk against him, he decides to sell out Harvey to save himself, instead. When Jessica finds out, she uses the information Donna gave her to blackmail Cameron into backing down.