Attack on Titan features one of the most dramatic hero-to-villain arcs in all of manga and anime.

　　Attack on Titan was first made famous by a relatively simple premise: in an alternate world from our own, humans must defend their walled home from giant, flesh-eating monsters who roam the Earth. It's essentially a zombie apocalypse story, but the setting is a historical one and the zombies range from around seven to several hundred feet tall. As Hajime Isayama's grim, survivalist story has expanded, the story has become an ever more twisted web of political intrigue, race wars and mind games.

　　This couldn't be better personified by its main protagonist, eren jaeger, who's arc through the manga has come to be one of the most dramatic the shonen genre has to offer. Once a young, vengeful teenager, Eren soon earns a reputation, in his colleagues' eyes, as one of humanity's best hopes for the future. Gradually, he grows into a hardened and divisive rebel leader in his early adulthood, before emerging, as of Chapter 124 of the manga, as Attack on Titan's most complicated anti-villain.

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　　EREN'S ORIGIN

　　Eren grew up in the Shiganshina district with his mother, Carla, his father, Grisha and his adopted sister, Mikasa. Along with their childhood friend, Armin, the trio's lives were forever changed when the Colossal, Armored and Female Titans brought down Wall Maria and devastated their home, forcing its citizens to flee. On a more personal level, Eren witnessed his mother being eaten alive in front of him, an experience that would steer the course of his life for years to come.

　　Another life-changing moment came when Grisha, amid all the chaos, entrusted his son with a key to the sealed basement under the Jeager home, and -- though Eren wouldn't come to remember it until much later -- injected him with Titan serum and forced his son to consume him. These acts granted Eren the Attack and Founding Titan powers. (And here begins the tale of one of the most messed up families in manga and anime.)

　　EREN'S INNER POWER

　　Even before Carla's death, Eren had always felt an irrepressible pull to journey beyond the confines of his home -- something he shared with Armin. The death of his mother infused that urge with a burning desire to wipe the Titans out in retaliation, leading to him, along with Armin and Mikasa, joining the 104th Training Corps.

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　　It was during this time that Eren discovered, to his horror, that his body housed the ability to become a Titan. The revelation was twinned with the exposure of one of his fellow soldiers in the 104th, Annie, as the Female Titan, leading to a showdown between the two, where Eren, as the Attack Titan, scored his first major victory.

　　Eren, Armin and Mikasa also become an integral part of the Battle of Trost District, where Eren performed what remains one of his most selfless acts: rescuing Armin from the slathering jaws of a Titan in his human form. In his Titan form, he was then able to fulfill Armin's hairbrained idea to plug up a breach in Wall Rose, saving another district from being destroyed.

　　Despite Eren proving himself a bonafide hero by this point, the military was, naturally, still highly suspicious of a possible enemy in its ranks, leading to Eren being put on trial more than once; the beginnings of a continually fraught relationship between Eren and the organization that molded him from a desperate child into a capable warrior. When asked by Captain Levi, "humanity's strongest soldier," what his goal was, Eren reaffirmed his desire was still pure and simple: to rid the world of Titans.

　　EREN'S NEW ENEMY

　　As if finding out that you are the very monster you swore revenge on wasn't confusing enough, Eren's time as a member of the Survey Corps came to muddy his already complicated view on Titans. As more of the Warriors -- humans who possessed the Nine Titan powers -- were exposed to be more of Eren's friends, the prospect of killing the creatures became a harder one to swallow. The vengeance mission Eren had been holding onto all these years also reached a point of dramatic closure when he happened to encounter the "smiling" Titan who'd devoured his mother.

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　　As his, by this point, trademark rage overtook him -- and his Attack Titan power failed him -- Eren was able to unwittingly channel the dormant power of the Founding Titan, compelling the surrounding Titans to kill his mother's murderer. With this chapter closed, others followed suit: Eren and his squad were later able to retake his home district back from the Titans and, more crucially, unlock the secrets of his father's basement.

　　There, they discovered the truth about their supposed isolation. The barrage of Titan attacks had a human source: the distant nation of Marley, who had been forcibly transforming Eren's kin (the Eldian Subjects of Ymir, the Founding Titan) into the human-eating monsters for years. All of this information marked a formative shift in Eren's motivation. No longer did he have a mother to avenge, a home to salvage or an easy, brainless target to aim for. Instead, his real enemy was the worst kind: other humans.

　　EREN'S DESCENT

　　The divide between Eren and the military, as well as his growing dispassion for other factions of humanity, was rendered startlingly clear when the manga skipped ahead four years into the future from the reclaiming of Shiganshina. While infiltrating Marley with the rest of Survey Corps, Eren strikes out on his own and delivers a crushing surprise attack as Marley's top brass gathers in one place.

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　　The gains of this act were huge: Eldia, Eren's nation, were able to snatch the War Hammer Titan's power and capture Eren's half-brother, Zeke, the holder of the immensely powerful Beast Titan power. However, the Marleyan civilian casualties were high -- something Eren uncharacteristically shrugged off as a necessary loss. From this point, all of Eren's unrelenting need to free his people from the Titans was now being channeled, instead, into freeing them from opposing human forces.

　　So committed was Eren to this goal, in fact, that he was willing to, superficially, throw away his friendship with Armin and Mikasa, become the figurehead of a vicious coo within the Eldian military and even align himself with Zeke, whose self-hating plan to "save" the Subjects of Ymir -- and the rest of the world -- was to use their Founder's power to sterilize them, thus ending their entire bloodline.

　　At the last moment, however, Eren turned on his half-brother, restating his loyalty to the people he loved. Crisis averted, right? Well, no. While Eren wasn't a fan of the horrible sterilization plan, he did have his own insidious agenda. His true intent all along had been to harness Ymir's power to awaken the thousands of ginormous Titans within the island's walls and trample the rest of the world to dust. Only then, he reasoned, could his people truly be free from centuries of persecution.

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　　It could be argued that Eren is somewhat absolved of a portion of the blame, here. When he confronts Ymir in the strange, supernatural dimension that her spirit is bound to, he gives her a choice, not an order: "You're no slave. You're no god. You're just a person. You don't need to serve anyone. You can be the one to choose. [...] Stay here for eternity... or end it all." He also implies that Ymir has been the one guiding him -- and every previous holder of the Attack Titan -- to this very point, across 2,000 years, in the hopes of liberation.

　　The Attack Titan is a unique power even among the Nine; one characterized by a resolve to always move forward, powered by its ability to see both its past and future lives. It's a characterization that, whether good or bad, has always defined Eren, too. Given that Ymir is the root of all Titans and Eren has been in possession of the Attack Titan since he was a child, the line between his and his Founder's goals could be blurred. Liberation is certainly what Ymir gets in the end, but, however sympathetic her plight is, this turn of events only perpetuates the cycles of vengeance that Attack on Titan's world keeps getting ripped apart by.

　　Whether Eren is acting entirely of his own volition or not, his journey from hopeful hero to genocidal god is one hell of a ride.

　　The Attack on Titan manga is available in English from Kodansha. New chapters are available on monthly digital release from Crunchyroll. The Attack on Titan anime will return for a fourth season in fall 2020, though the show's exact release date has not yet been announced. It's unknown at this time if Season 4 will be split into two halves.