Agent Kallus aka ‘ISB-021’ aka Fulcrum, from Star Wars Rebels, is an undercover spy working for the Rebels.
He belongs to the law enforcement agency of The Galactic Empire called the ‘Imperial Security Bureau’.
Before defecting to the Rebels, he is a very strong believer in the Galactic Empire and its rule of law. He tries his best to corner, capture, kill and eliminate the Rebels.
Then something happens that makes him change his mind and join the Rebels.
Garazeb Orrelios aka Zeb is a Rebel from the well-known Rebel starship Ghost. In one encounter between Kallus and Zeb, Kallus almost killed Zeb, but he is saved by Ezra Bridger.
Later on Kallus tries to capture ‘Ghost’ and its crew on the planet Geonosis but fails. Kallus and Zeb crash-land on the moon Bahryn.
The moon has a very harsh frozen climate with deadly predators.
In order to survive, the duo cooperate with each other. They survive the moon and its deadly inhabitants until they are rescued.
After the incident, Agent Kallus develops a kind of respect for Zeb and a liking for the Rebels because Zeb has very high moral values. He questions his beliefs regarding the Galactic Empire and its methods.
After Grand Admiral Thrawn of The Galactic Empire kills Morad Sumar (a farmer on Lothal), Kallus realizes that the Empire is really evil.
In this moment of truth he makes the life changing decision of joining the Rebels. He does this because his assumptions about the Empire and The Imperial Cause are shattered. He starts believing that the Rebels’ cause is worth supporting.
From this point onward, Kallus uses his abilities to help the Rebels fight the evil Empire. He spies on the Empire and tries to relay helpful information to the Rebels.