Did high level Imperial officers join the Rebellion?

Question

Looking at Earth (war in Syria, or Libya before that), modern rebellions seem to be often based on having high level generals defect from the government forces to the Rebels.

But everyone in Star Wars rebellion who was an Imperial before, was a civilian (Senators Mon Mothma, Leia Organa, Bail Organa).

Did high ranking Imperial officers join the Rebellion? Or were they really 100% loyal to the Emperor? The only one I could think of, General Dodonna, wasn't an officer when he joined (he had retired from the Imperial military). Admiral Ackbar was a slave to Tarkin.

Answer 1

• The most prominent I recall was General Crix Madine, creator of Storm Commandos. I think the Imperial rank he attained was Commander.

  While most details about him come from C-canon, the character himself appeared in G-canon (participating in Episode VI):

  

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• Baron Soontir Fel was a legendary TIE-fighter pilot (and without knowing it, brother-in-law of Wedge Antilles). He defected to the Rebellion at Brentaal IV (couple of months after Endor).

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• Vin Northal wasn't literally high ranking... but he was a member of the Emperor's Royal Guard

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• Brenn Tantor was an Imperial General who defected.

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• Kyle Katarn's exact Imperial rank is unknown but he was a highly regarded officer before defection.

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• If you go after Battle of Endor, we have Sair Yonka. Unlike the rest of the upstanding citizens on this list, this Star Destroyer Captain defected after Rogue Squadron offered him money.

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• Another one was General Pashna Starkiller (though I'm not sure if he counts here, since he was retired before joining the Rebellion, like Dodonna). Same as Madine, he was mostly covered in C-canon but was from G-canon (present at awards ceremony in ANH).

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• Trey Callum's Imperial rank isn't known, but he was a Sergeant Major in the Rebel forces on Hoth.

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• Admiral Harkov tried to defect; but failed. Same with Admiral Joser

Answer 2

In the official Disney canon a few have.

• Sinjir Rath Velus - Formerly in the position of the feared loyalty officer, Rath Velus left the Empire after the Battle of Endor. He then began a life as a drunk. However, a y-wing pilot and her son convinced him to join the Rebel Alliance. He joined them, as well as a few others, in becoming a squad that captured Imperial officers. (Aftermath trilogy)
• Kallus - (Speculative, although its pretty obvious) Kallus was an officer of the ISB during the early days of the rebellion. After learning of the Empire's various wrongdoings (specifically the extermination of the Geonosians), he became an informant, telling the rebellion of a small cell of rebellious TIE pilots. (Star Wars Rebels Season 3)
• Belkor Dray - Dray was an Imperial colonel on the planet Ryloth during the early years of the Empire. He reluctantly assisted the Free Ryloth movement in destroying a star destroyer, and later continued to feed the movement information. His true goal in this was attempting to overthrow his boss, Moff Delian Mors, and take her position. However, he was discovered, and was summarily killed. (Lords of the Sith)
• Dodd Rancit - Rancit was an officer of the Naval Intelligence Agency five years into the Empire's reign. He assisted Berch Teller's rebel cell in stealing Moff Tarkin's ship, the Carrion Spike. He was planning on capturing them and being promoted because of it. However, his plot was foiled by Darth Vader and Moff Tarkin, so he was executed. (Tarkin)
• Cogon and Swain - Swain and Cogon were two young officers of the Empire who attempted to join a rebel cell on Klonoid. They almost escaped the Empire on the planet Thral, but were captured and tortured. (Star Wars Rebels Magazine #14 "No Sympathy")
• Thane Kyrell - Designated as AV-547 when in the Imperial Navy, Kyrell joined the Rebel Alliance after witnessing the destruction of Alderaan. He fought with the Alliance for years, all the way to the Empire-ending Battle of Jakku. (Lost Stars)
• Kendy Idele - Idele was friends with Kyrell and was a roommate of his girlfriend, Ciene Ree, during their years at the Imperial Academy. Idele also became a TIE pilot and defected to the rebels. (Lost Stars)
• Wedge Antilles - Surprising as it may be, Wedge Antilles was in fact an Imperial cadet before joining the Alliance. Two years before the Battle of Yavin, Antilles was recruited by the Empire to fly TIE interceptors. He defected after realizing the Empire is all-too-willing to kill innocents.
• Derek "Hobbie" Klivian - Klivian was recruited at the same time as Antilles and defected at the same time as well, for the same reasons. Nothing else much to say here.
• Galen Erso - This will be spoilery. You have been warned.

Erso sent a holomessage to rebel extremist Saw Gerrera, which revealed that he purposefully built the exhaust port to be destroyable.

• K-2SO - K-2SO was an reprogrammed Imperial KX-series enforcer droid that served in the squadron Rogue One. That's all I can say without getting too spoily.

There are most likely more, but none that I can think of off the top of my head.

Answer 3

yes, I mean from a basic perspective with a base knowledge in what was once cannon (now to be changed) a lot of the rebel alliance i.e. pilots,troopers and so forth were often defectors from the empire. look at darklighter and others and the trend is rampant.