Whenever betrayals in 'Bleach' come up in my feed, I end up ranting about how savage the Arrancar arc was — it’s the one where captains actually turning their coats became a major plot engine. The big, clear-cut departures from the Gotei 13 during that period were Sōsuke Aizen (then captain of the 5th Division), Gin Ichimaru (captain of the 3rd Division), and Kaname Tosen (captain of the 9th Division). Aizen’s betrayal is the centerpiece: he faked his own death, revealed his experiments and ambitions, and basically left Soul Society to build his own power base in Hueco Mundo. Gin and Tosen followed him for their own complicated reasons — Gin out of a long game against Aizen and Tosen because of his twisted sense of justice — and their leaving shattered the expected stability of the captain corps.
If I step back a bit, there’s another important nuance fans sometimes overlook: several prominent characters had already left Soul Society long before the Arrancar conflict and only reappear during later arcs. The Visored, for example, are ex-captains and lieutenants who left the Gotei ages earlier after experimenting with Hollowfication; Shinji Hirako is the poster child for that group. Those departures weren’t part of the in-story betrayal scene in the Arrancar arc, but they do affect how you view the captain lineup when the series shifts into the bigger conflicts. So if someone asks “which captains left?” you really need to separate: (a) captains who defected during the Arrancar storyline and (b) former captains who had left earlier and showed up later as outsiders.
Later arcs like the 'Thousand-Year Blood War' shake things up in different ways — players die, new captains step up, and the roster changes — but voluntary, dramatic walkouts like Aizen-Gin-Tosen are what people usually mean when they say captains ‘left’. I still get chills remembering how personal those betrayals felt in the manga: it wasn’t just political, it was intimate, like friends turning into enemies, and that’s why those moments stuck with me. If you want, I can list who replaced those captains or map the timeline of ex-captains versus defections next.