WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 22, "Thaw," now streaming on Funimation, Hulu and Crunchyroll.
The Rumbling is underway in Attack on Titan as the full scope of Eren's world-ending plan is still unfolding. After the tattered remains of the Survey Corps got the Titan situation in Shiganshina under control, the Rumbling threw another curveball at them in the form of an old foe. When Armin Arlert learned from Gabi Braun that even Reiner's Titan's armor shattered when Eren activated the Rumbling, he quickly realized who else would be affected by Titan hardening coming undone. Encased in an impenetrable shell since the end of Season 1, Annie Leonhart has been forcefully freed from her self-imprisonment.
Annie Leonhart, the holder of the Female Titan, was the ultimate antagonist of AoT's premiere season and the first definitive proof that humanity was not extinct beyond the Walls. After infiltrating the military alongside Reiner Braun and Bertholdt Hoover, Annie was the first to have her cover blown as she attempted to abduct Eren Jaeger for reasons unknown (at the time). After her defeat at the hands of Eren and Mikasa, Annie sealed herself away with her Titan's hardening ability before someone like Erwin Smith, aka the GOAT, could get his hands on her for interrogation. And it is inside that shell she has stayed for years as the war between Paradis and Marley escalated.
The world of Attack on Titan has changed dramatically since Annie was last awake. Bertholdt is dead, Zeke has betrayed Marley and Reiner has been a hot mess for a while now. The Survey Corps is all but extinct, as Eren Jaeger's supporters overthrew the very military that was keeping an eye on Annie's slumber deep underground. Annie's loyalty to Marley will certainly be tested once she gets her wits about her, considering Marley has much bigger problems at hand than her whereabouts -- and if she wanted to get away from them, now would be a good time.
Alliances have been shifting during the war, even more so in the brief time since the Rumbling began, so it's unclear who will have Annie's support if she cares to get involved in Eren's latest and greatest mess. Armin, who is harboring a crush on Annie -- possibly a result of Bertholdt's own feelings -- should be more inclined than anyone to track her down before she gets too far. His personal feelings aside, Armin doesn't appear to be on board with Eren's wholesale slaughter of all mankind and will know they're going to need all the help they can get -- and someone who possesses one of the Nine Titans would be a tremendous asset.
When Annie first attacked the Survey Corps to retrieve Eren, she took a near-sadistic pleasure in the slaughter of the Scouts, more so than almost anyone else from Marley. As a result, there may not be anyone on Paradis Island other than Armin willing to trust her. Reiner looked to be down for the count, Pieck vanished in the chaos of the Rumbling and Zeke was taken by Eren, so Annie doesn't have many options in terms of allies. If nothing else, it will be interesting to see what Annie does with her newfound freedom, even if that freedom might have come right at the end of the world.
Attack on Titan has come a long way since Annie was last a major character, and as evidenced by the genocidal maniac called Eren Jaeger, people can change. Whether Annie has changed while essentially being in stasis for the majority of the series is unclear, but her reappearance now likely foreshadows major involvement in the series' endgame. Eren's friends are still sorting out how they feel about what he's done, but that none of them seem thrilled by it might mean an alliance to stop Eren is on the horizon. However, Annie does have one major reason to want to stop Eren: her father lives in the Liberio Internment Zone, and the only people in the whole world who are safe from the Rumbling are the people currently on Paradis Island.
