Steven Haladki was a pompous CIA agent. He was also secretly working for Alexander Khasinau.
Season 1
Haladki was seen first entering Vaughn's officer shortly after the CIA figured out that Sydney's mother was the one who murdered a dozen CIA agents, one of whom was Vaughn's father. Haladki asked Vaughn if he was interested in dating another agent who was available, but Vaughn declined the offer. (The Box, Part 1) Haladki had a personal vendetta against Vaughn because he realized that he was becoming emotionally attached to Sydney and always went by the book. Even when Vaughn went into SD-6 alone during a raid executed by McKenas Cole, Haladki still didn't even offer Vaughn any backup and simply tried to explain to his surperiors why he should be fired. (The Box, Part 2)
Later, Haladki convinced the DSR not to ignore a warning seen within Page 47 and they later took Sydney into custody after discovering what the prophecy stated. (The Prophecy) Despite their hatred for each other, Vaughn tried to become friends with Haladki, but he refused and continued to pursue his connection with Sydney Bristow. (Q&A)
After finding out that Haladki didn't have clearance for knowing about The Circumference, Jack figured out that not only was Haladki the mole that everyone was looking for, but that he was working for Khasinau. Jack brutally interrogated Haladki by crushing his hands between gears and Haladki revealed he had been working with Khasinau for two years. Haladki also told Jack that he needed the Circumference for a battery and that he told Khasinau Sydney was a double agent. Jack, infuriated that Haladki risked his daughter's life, killed him by shooting him in the head. (Almost Thirty Years)