Karl Dreyer was an Alliance of Twelve agent who specialized in giving agents lie detector tests.
After SD-6 realized there was a mole somewhere in the organization, Sloane contacted Dreyer so he could give Sydney a lie detector test. When the results came in, Dreyer told Sloane that he believes Sydney's the mole (Time Will Tell).
When Sloane reviewed Sydney's results, he noticed Sydney passed with a perfect score, and told Dreyer that he was a desperate man trying to put the blame on someone. Dreyer, however, thought that her results were too perfect and that she was the mole anyway; he used Sydney's recent reckless actions (such as Dixon nearly being killed, and the loss of Rambaldi's journal) to justify his reasons. Nevertheless, Sloane told Dreyer not to do anything (Mea Culpa).
Later on, Sloane confirmed that Sydney wasn't a mole after he ordered an assassination attempt on her. However, it was all a test; there was no hit in place. Sloane just wanted to see if the CIA would extract her, and they didn't. After Dreyer found out, he scolded Sloane, saying that if Sydney had been a mole, SD-6 and the Alliance would've been severely compromised. Ironically, after Sydney was sent to Geneva for another mission, Marshall came straight to Dreyer and told him there was a second transmission coming out of the area that wasn't theirs. Dreyer later handed Marshall's report to Sloane, and Sydney was captured for further questioning (Mea Culpa).