At first, Gin seemed like an active supporter of Aizen’s treacherous schemes. He was one of the only shinigami captains to defect and stayed by his master’s side throughout the Espada arc. There was even a time when he fought Ichigo as a significant threat.
However, everything Gin did was in order to get himself closer to Aizen. At the height of the villain’s power, he pierced his Hogyoku while touching his Zanpakuto to negate its hypnotic effects. Unfortunately, Gin’s treachery was not enough, and he died in vain.
Captain Yamato was instrumental in preserving the Leaf. His rare usage of Wood Style helped to keep Naruto under control while mastering Kurama and rebuilding Konoha after Pain’s invasion. However, his origins were less than stellar.
Yamato was a former experiment of Orochimaru that got recruited into Danzo’s seditious Foundation program. Fortunately, Kakashi helped to rehabilitate him into a much more noble path than the one initially chosen for him.
Reiner (Attack On Titan)
Technically, Reiner Braun never changed sides. He was always loyal to Marley regardless of how badly they mistreated him. However, after the Rumbling began and Eren announced his plans for global genocide, the definition of “hero” and “villain” had a noticeable shift.
As a result, the man who was once a staunch enemy of Paradis became humanity’s last hope from utter annihilation. Despite how his conscience weighed on him, Reiner mustered the strength to fend off pure titans while Armin and Mikasa dealt with Eren.
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Satsuki (Kill La Kill)
Satsuki Kiryuin was tasked with spreading life fibers across society as much as possible by Ragyo. This was to make the impending takeover much more seamless, which would result in the destruction of the planet. Though Ragyo abided by her mother’s request, it was for a much different reason.
Instead of using the life fibers to destroy humanity, she hoped to strengthen it enough to prevent her mother’s onslaught. After being exposed, Satsuki was immediately imprisoned and tormented for her defiance. In the end, only Ryuko herself could save the day.