Eren Jaeger lives within a cage, constantly awaiting, whether it be consciously or unconsciously, his own demise at the hands of beings that wish to harm him. Those beings can be complete outsiders, people on the inside with him, his allies, even his friends, and this first season of Shingeki no Kyojin illustrates this well.

The anime adjusts the manga content as to help focus more on who Eren's allies are, telling the story chronologically as opposed to the manga's constant time jumps. We see Eren's hometown get destroyed, the birth of his resolve, that resolve hardening through his training, and his re-encounter with the Colossal Titan all in an order that follows Eren's own growth and development. While this change can be argued as having weakened the pacing of these early parts of the series, as well as making some fodder deaths feel even more pointless (and believe me, I have stated these ideas on occasion), this change also allows the series to focus more on Eren's relationships.

From the start we get to see his friendship with Mikasa and Armin, during his training his rivalry with Jean is far more organic, his small bonds with Reiner, Bertolt, and Annie among others are given more prominence through this avenue. We meet all these characters, and then the story really begins, allowing the viewer to keep these relationships in mind while watching the anime, and it arguably makes the end of season reveal about the female titan more impactful. This is your spoiler warning.

Annie Leonhart is the first of a number of infiltrators within the walls of Eren's cage. She situates herself as a member of the the Military Police within the furthest Interior Wall, and it is revealed later on in the narrative that she is the female titan. The female titan who has been killing the Survey Corps recently, the female titan who five years prior led the large group of titans to Eren's hometown, and now she is trying to capture Eren himself for reasons unknown. Despite the perceived bonds that Eren, Armin, and the others had made with her, she is just as easily able to work against them, but this betrayal has only made her mission all the more difficult. The person who she might have pretended to be left a strong impact on everyone in her training regiment, including (and especially) Eren Jaeger.

During their confrontations, Eren uses techniques she taught him during their training, only this time it's against her in a real battle. Eren is quite literally taking this person who betrayed his trust and tried to take his minimal amount of freedom, and is trying to defeat or even kill her. This is the start of a trend of Eren taking people into himself, metaphorically and eventually literally, and this betrayal will leave a mark on the cast for the series to come.

Finally, once the battle concludes and Annie seals herself away, one final reveal is thrown. While Eren had been in a cage, and he hated the mere fact of that idea, he had still believed the cage was a wall of protection. But, Annie's hasty failed escape only revealed that their enemies are on the inside as well, and perhaps even more terrifying than the ones on the outside, a perfect lead in to the following seasons of Shingeki no Kyojin.

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