Gar Saxon is the Imperial Viceroy of Mandalore and leader of the Imperial Super Commandos, introduced in Star Wars Rebels. StarWars.com describes him as "Gar Saxon of Clan Vizsla", "the ruthless leader of the Imperial Super Commandos — Mandalorians loyal to the Empire", in armour with "a threatening color scheme of red and white, with the Imperial cog emblazoned on his shoulders".
Collaboration, worn openly
The cog is the whole characterisation. Saxon does not disguise the arrangement or pretend to be an independent actor: he is a Mandalorian who has accepted that the way to hold Mandalore is to hold it for somebody else. His Super Commandos are the Empire's answer to the problem of policing a warrior culture — recruit the warriors.
Against Clan Wren
His campaign against Clan Wren is the engine of Rebels' third and fourth seasons and the reason Sabine Wren finally goes home. He is killed by Ursa Wren after attempting to shoot Sabine in the back during a duel he had already lost — a death staged to make the point that the collaborator has abandoned the code as well as the planet.
What follows
His brother Tiber Saxon inherits the viceroyalty and the weapon Sabine designed, and the Duchess arc that closes out her story is a direct consequence. Gar Saxon matters less for what he does than for what his existence proves: the Empire did not conquer Mandalore so much as hire it.