One of Luther Sloan's two aides in the staged Internal Affairs investigation of Julian Bashir in 2374, who acts as the doctor's hostile guard.
Kagan appears in the Deep Space Nine episode Inquisition. He arrives with Sloan and Chandler in the uniform of a Starfleet security officer and is given the job of keeping watch over the senior staff, and then over Doctor Bashir in particular.
Where Chandler is courteous, Kagan is openly contemptuous. Once Sloan accuses Bashir of spying for the Dominion, Kagan treats him as a traitor, marching him through the Promenade in restraints and making his dislike plain.
The pressure is deliberate. The investigation is a Section 31 test conducted in a holographic copy of the station, and Kagan's hostility is one of the tools Sloan uses to see whether Bashir will crack.