Zack Allan is a character in the fictional universe of the science-fiction television series Babylon 5, played by Jeff Conaway. He regularly appeared in the show from season 2 onwards. During the second and third seasons of the series he was a security officer on the Babylon 5 station. In the fourth season he was promoted to become Babylon 5's Security Chief, and he maintains that position through the end of the series fifth and final season.
Briefly in season 2 and 3, Allan was a member of Nightwatch, the sinister police force created by Earth Alliance President Morgan Clark to expose and arrest "traitors to Earth". True to form, Zack joined Nightwatch for the bonus pay they offered, without paying much attention to their political views and fascist nature. However, he had the truth of their nature driven home when a shopkeeper accused of "sedition" (i.e., publicly criticizing Clark's Presidency) was physically dragged away from his store and imprisoned by Nightwatch officers in the episode, The Fall of Night. After this experience Zack agreed to help Babylon 5's command staff, eventually playing a critical role (in Point of No Return) in leading the bulk of the station's Nightwatch into a trap set up to capture them.
Allan was the second aide to B5 Chief of Security Michael Garibaldi from 2259-2261. Following Garibaldi's resignation as chief of security in season 4, Allan was appointed Chief of Security. He held the position for many years; except for a brief stint back on Earth, he remained there until the station was decommissioned in 2281. Because of this, he was the only one not to receive his invitation to Sheridan's farewell party in Sleeping in Light, though Allan would still meet Sheridan one more time as Sheridan took one last walk through the station.
In Sleeping in Light, he walked with a noticeable limp, which has never been explained on screen. In the voiceover commentary of the episode, series creator J. Michael Straczynski explained that Zack was involved in heroic activity and lost his leg. Later in the episode, after the aged and abandoned Babylon 5 was destroyed and the Drakh influence on Centauri Prime was exposed and eliminated, Zack joined the Rangers and became Centauri Emperor Vir Cotto's assistant and (presumably) a direct liaison between the Emperor and the Interstellar Alliance.