Kallus was a human male Agent of the Imperial Security Bureau, operating number: ISB-021, in the years prior to the Battle of Yavin. Kallus, a firm believer in the Imperial cause and bringing order to the galaxy, was tasked with suppressing resistance against the Galactic Empire before such acts could spark a full-scale rebellion. He was also involved in the cleansing of the planet Lasan and the near-extinction of the native Lasat species, though he would later turn out to be a key part of a prophecy that promised a new start for the Lasat race.
Kallus was later assigned to the Outer Rim planet of Lothal, where he pursued the rebels who operated there in an attempt to stop them from assembling a larger rebel movement. His frequent efforts, along with those of the Grand Inquisitor and government officials, were unsuccessful in stopping the rebellious activities, drawing the attention and ire of Grand Moff Tarkin. Under the supervision of Tarkin, Kallus and The Inquisitor succeeded in capturing the former Jedi Padawan Kanan Jarrus. However, Kanan escaped due to the help of his rebel comrades, the mysterious rebel leader Fulcrum, and the Phoenix rebel cell.
Kallus later assisted Darth Vader's efforts to destroy the Lothal rebels. During that attempt, he engineered the death of the Imperial minister Maketh Tua in order to turn the people of Lothal against them. Even after the Lothal rebels had fled Lothal, Kallus continued hunting them and even joined forces with other Imperial servants like Admiral Kassius Konstantine, the Fifth Brother, and the Seventh Sister. Despite their best efforts, the Lothal rebels always escaped their traps and continued to pose a threat to the Empire. In addition, Kallus and his Imperial colleagues were also forced to deal with the growing strength of the rebellion, an underground movement seeking to overthrow the Empire.
During an attempt to ambush the Lothal rebels on Geonosis, Kallus found himself trapped in an escape pod with Zeb Orrelios, a Lasat member of that rebel cell and a survivor of the Fall of Lasan. Despite being long-standing enemies, the two were able to forge an unlikely friendship by working together to survive on the frozen moon of Bahryn. Kallus' friendship with Orrelios forced him to question his beliefs and assumptions about the Empire and the rebellion. As a result, he eventually became a Fulcrum agent, providing insider intelligent to the Rebellion and aiding Sabine Wren, Kanan, and Ezra Bridger on missions. Eventually he was discovered by Grand Admiral Thrawn and tricked into helping reveal the location of the Phoenix Squadron base, but later escaped and was rescued by the Rebels.