As an agent of Imperial Security Bureau, Kallus was a longtime enemy of the Ghost crew, had fought rebels like Saw Gerrera, and even participated in the genocide of the planet Lasan. These actions obviously hurt the Alliance, but his loyalties to the Empire began to shift after he found himself stranded
on a frozen moon with Ghost crew member Zeb. By working together to survive and through their conversations, Kallus began to question his beliefs in the Empire and the Rebellion, ultimately leading him to become a rebel spy known as Fulcrum. In this capacity, Kallus provided the Rebel Alliance with crucial intelligence on the Empire, especially concerning Grand Admiral Thrawn. Even after his cover was blown, Kallus managed to escape the Empire and continued serving the Rebellion on Yavin 4, and later on helped liberate Lothal from the Empire.