- "Our scout ships have reached Dantooine. They found the remains of a Rebel base, but they estimate that it has been deserted for some time. They are now conducting an extensive search of the surrounding systems."
- ―Cass reports to Grand Moff Wilhuff Tarkin
Siward Cass was a male human who served the Galactic Empire as an Imperial officer. Fifteen years into the Emperor's reign, Cass held the rank of major, and he served as an aide to Grand Moff Wilhuff Tarkin, governor of the Outer Rim Territories. At the time, there were rumors that Commandant Brendol Hux had created a secret society within the Arkanis Academy. On Tarkin's order, Cass transferred Lieutenant Chiron from the Academy for Young Imperials on Lothal to Arkanis so he could investigate the matter. The major notably made sure that Chiron's code cylinder would give him access to Area Null, a restricted part of the academy where the Empire ran a number of secret programs.
Major Cass was still serving under Tarkin during the Galactic Civil War, which pitted the Empire against Alliance to Restore the Republic. Shortly before the Battle of Yavin, Cass attended a meeting on board the Death Star in which he witnessed Admiral Conan Antonio Motti and General Cassio Tagge argue about the station's power, before the Sith Lord Darth Vader Force choked Motti. He later reported to Tarkin that the planet Dantooine was not the location of the main Rebel base. Not too long afterward, the Battle of Yavin commenced, with the Alliance destroying the Death Star battle station.
- Servants of the Empire: The Secret Academy (Mentioned only) (First identified as Cass)
- Star Wars: Episode IV A New Hope (First appearance)
- Star Wars: A New Hope junior novelization
- Star Wars: The Original Trilogy – A Graphic Novel