Thrawn used to have a mutal respect for Kallus, and regarded him as a loyalist to the Empire. However, in secret, Kallus was a Fulcrum spy.In truth, Kallus was scared of Thrawn and refused to fail him so he could keep up appearances that he ewas still loyal to the Empire anda not to the Rebellion.
In "Through Imperial Eyes," Thrawn eventually learned who Kallus was after Kallus protected Ezra Bridger's identity of their missing prisoner. Thrawn later use Kallus to get to Atollon and lost his prisoner when Kallus escaped from his guards.