Alexsandr Kallus is a human Imperial Security Bureau agent from Coruscant, voiced by David Oyelowo in Star Wars Rebels from the 2014 pilot film Spark of Rebellion onward. He begins the series as its antagonist-in-residence: the officer sent to Lothal to destroy a rebel cell that keeps embarrassing him.
Lasan
The show establishes early that Kallus took part in the Imperial assault on Lasan and carries a bo-rifle taken from a Lasat Honor Guard. That single prop does most of the work. It makes him personally, not institutionally, responsible to Garazeb Orrelios, one of the last survivors of that world, and it means every conversation between the two men has a body count under it.
Bahryn
The turn happens in one episode. Kallus and Zeb are stranded together on the frozen moon Bahryn and have to keep each other alive. Nothing is forgiven; the episode is careful about that. What changes is that Kallus is forced to spend a night with a person rather than a target, and afterwards he cannot go back to the version of the galaxy he had been operating in.
Fulcrum
He becomes a rebel informant, using the Fulcrum codename, and spends most of season three feeding intelligence out of an Imperial command structure while Grand Admiral Thrawn methodically narrows the field of suspects. It is the best sustained spy plot the animated shows have done, and it ends with Kallus deliberately framing Lieutenant Yogar Lyste to buy himself time — a reminder that a change of side is not a change of trade.
Exposed
Thrawn works it out. Kallus is caught, escapes, and defects outright, ending the series as an Alliance officer at the liberation of Lothal — where, in a piece of construction the show clearly enjoyed, he is the one who throws the switch that kills Thrawn's assassin Rukh.
Why he is the best thing the ISB has
Star Wars has a standing problem with Imperial characters: they are usually either irredeemable or they defect in a single scene. Kallus takes four seasons, and the show never lets him off. He is not forgiven by the people he hurt, he does not get a speech, and his usefulness to the Rebellion is precisely the skill set that made him monstrous. It is the most honest redemption arc in the franchise.
Where to watch him
Star Wars Rebels, all four seasons, in order. The relevant hinge episodes are "The Honorable Ones" (season two) and "Through Imperial Eyes" (season three).