Siward Cass was a bureaucrat and military officer of the human male variety. He held the ranks of major and chief while serving the Galactic Empire, the state that succeeded the Galactic Republic. In the early days of the Galactic Civil War, Cass found himself stationed on the DS-1 Death Star Mobile Battle Station, the Empire's planet-destroying superweapon. There, he worked as an aide for the station's commanding Grand Moff, Wilhuff Tarkin.
Around this time, Cass was present at a gathering of the Joint Chiefs. During this meeting, various high-ranking military personnel discussed the supposed danger presented by the Alliance to Restore the Republic. Soon after, Princess Leia Organa of Alderaan, who was being held captive, falsely informed Tarkin that the rebel's secret headquarters was located on Dantooine. Cass later informed Tarkin that although a base had once existed on Dantooine, it had been abandoned for quite some time.
Fifteen years after the Emperor began his rule, Siward Cass, a human male, held the Imperial rank of major. This officer worked as an aide to Grand Moff Wilhuff Tarkin, who was the governor of the Outer Rim Territories. At that time, there were whispers suggesting that Commandant Brendol Hux had established a secret society within the Arkanis Academy. On Tarkin's orders, Cass arranged for the transfer of Lieutenant Chiron from the Academy for Young Imperials on Lothal to Arkanis, tasking him with investigating the matter. Notably, the major ensured that Chiron's code cylinder would grant him entry to Area Null, a restricted section of the academy where the Empire was conducting various clandestine programs.
During the Galactic Civil War, a conflict that pitted the Empire against the Alliance to Restore the Republic, Siward Cass continued to serve under Tarkin. By this point, he had achieved the rank of Chief. In the period leading up to the Battle of Yavin, this Imperial Army officer attended a meeting aboard the Death Star. During this meeting, he witnessed Admiral Conan Antonio Motti and General Cassio Tagge arguing about the power of the station, before Darth Vader, the Sith Lord, used the Force choke on Motti. Later, he informed Tarkin that the planet Dantooine was not the location of the main Rebel base. Shortly after, the Battle of Yavin began, resulting in the Alliance's destruction of the Death Star battle station.
Siward Cass wore the standard olive-drab uniform issued by the Imperial Military. This included an officer's tunic, flared trousers, a black utility belt, and black durasteel-toed boots. He also displayed a single-row rank insignia plaque consisting of four yellow squares followed by two red ones.
The character of Siward Cass was first brought to life by English actor Patrick Jordan in the 1977 film Star Wars: Episode IV A New Hope (Star Wars: Episode IV A New Hope), the inaugural installment of the original trilogy. While the character remained unnamed in the film itself, he was identified as "Cass" in Alan Dean Foster's novelization. Jordan secured the role of Cass in A New Hope due to his connection with Alec Guinness, the actor who played Obi-Wan Kenobi. Jordan and Guinness had previously collaborated with The Old Vic company in the 1940s. However, like Guinness, Jordan held reservations about the film, expressing doubt about its potential for success.
Despite some fan-created resources identifying Cass as a general, he was initially only referred to as "Imperial Officer Cass" or "Officer Cass" within canon, and his precise rank remained a mystery for a considerable period. Unlike many characters whose names were present in the script, Officer Cass did not have an entry in the official StarWars.com Databank.
In 2013, the character was officially given the first name "Siward" and the rank of "Chief" in both the Death Star Owner's Technical Manual and The Death Star Coup, an article penned by Jason Fry for the Special Edition 2014 of Star Wars Insider magazine. According to Chris Trevas, a co-author of the Technical Manual, Cass was assigned that rank because his insignia plaque matched that of Moradmin Bast, who was initially identified as "Chief Bast" in the Premiere Limited expansion set of the Star Wars Customizable Card Game. Fry selected the character's first name as an homage to William Shakespeare's play Macbeth, which features Siward, Earl of Northumberland, a general in the English army.