Ye Be Warned: There be spoilers in this post. Like, a lot of them
For real, major spoilers for every major plot point and twist in Batman: Arkham Knight follow. You have officially been warned, and should read no further without expecting spoilers.
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Excellent. So, how about that Arkham Knight? Yes, the titular character under the mask was exactly who many expected him to be since the beginning: Jason Todd.
The second Robin following Dick Grayson, who graduated to his role as solo hero Nightwing, the comic book version of Jason Todd was killed by the Joker after fans literally voted to axe him via 900 numbers. He returned either because Superboy Prime punched the walls of reality, or because Talia al Ghul revived him via a Lazarus Pit. Depends on when depends on which comics you consider cannon. Continiuity keruffles aside, Todd came back and donned the moniker of Red Hood. As Red Hood, Jason tried to out-Batman Bruce Wayne, showing him that the more permanent option (you know, killing criminals) was the better one.
It all eventually came down to Jason at least somewhat forgiving his surrogate father, donning the Bat-symbol once more, and working on the relative side of the angels.
The game follows a similar path, with the interim time period being spent as the Arkham Knight instead. Through trippy and terrifying flashbacks, we see Jason, as Robin, being tortured by The Joker for well over a year. In that time, he fakes Jason’s death on a tape sent to the Dark Knight, and continues to berate and beat him, blaming Batman for it the entire time, until he has Jason truly believing it’s Batman’s fault, and hating him for it. Jason Todd returns to Gotham, organizing a massive plan alongside Scarecrow and amassing an army to destroy everything Bruce Wayne cared about.
Once the two had their big confrontations, though, and Todd got rid of the proto-bat helmet part of his costume, he was left with just his tech helmet, shaded red, and looking like his comic book Red Hood costume. Later he’d even draw on the Bat on his chest before coming to the rescue, which was apparently as much redemption as he needed, despite the deaths and destruction he was responsible for. Some post-endgame chatter from thugs indicates that he’s running around taking many of them down (a wink and a nod to the story DLC for Red Hood coming later).
Ultimately, his journey in the game as the Arkham Knight wasn’t too much different from that in the comics where he was just the Red Hood the whole time. The primary difference was that he never actually died, and the torture at the hands of the Joker certainly brought things to a much darker place than the comics even dared to go. Ultimately, it all came down to Jason blaming Batman for not killing Joker any chance he got, and he threw a bit of a tantrum. A tank-driving, gun-toting tantrum.
Then there’s that Joker. While he was truly dead after the events of Arkham City, (his body was literally cremated) his presence drove much of Batman: Arkham Knight. The story revealed that there are five people in Gotham infected with Joker’s high-end new virus before he died, and one of them...is Batman. Through this device, the Joker is able to harass Batman throughout the game, terrorizing him from inside. Combined with exposure to the Scarecrow's latest fear toxin, the Joker appears as an extended hallucination during the game. Sometimes he showed Batman what could happen, while other times he showed Batman what has happened. But more than anything, he often just flat-out taunted Batman with his impending doom.
Eventually, the virus will run its course, leaving no more of Bruce Wayne/Batman, and only the Joker. It was a brilliant way to include the Clown Prince of Crime after his death, and somehow as a purely psychological threat he was scarier than ever. Aspects of The Joker manifested in the other four victims, as well, but when Batman’s eyes glowed green, it meant he’d done the unthinkable: he had lost.
Eventually, of course, Batman does emerge victorious, locking The Joker away in a dark, vaulted corner of his mind, but not before some possibly irreparable damage was done. He lied to and tricked Robin multiple times, dismissed Nightwing, and left Oracle to fend for herself. He lost Jim Gordon’s respect and eventually lost his secret identity, too. The whole game showed what a psychological master The Joker was, and it was better than any hand-to-hand fight could’ve been.
Ultimately, Batman: Arkham Knight took two well-explored tropes, the ways that other characters like Jason Todd (and The Wrath and Prometheus and Hush, etc, etc) can mirror Batman in training and skill, while The Joker always showed just how close to a true psychotic break-down Batman always sat. As all great Batman stories do, this one used the villains to explore aspects of Batman, and help us understand why the Dark Knight continues to rise no matter what.