The Arkham Knight was a mysterious military villain who emerged in Gotham City and was the antithetical version of Batman. He wore a militaristic "Batsuit" and had an Arkham symbol on his chest. He formed an Alliance with Scarecrow and struck a deal with Deathstroke to transform Gotham into a city of fear and end Batman.
The Arkham Knight was originally Jason Todd, the second person to take the mantle of Robin after Dick Grayson and before Robin. He was thought to have been killed by the Joker. Jason had always possessed an explosive temper with numerous incidents of him going too far in his actions as Robin, despite his natural talents and aptitude under Batman's guidance, with hints in his behavior that he might one day kill instead of using restraint.
One day, according to a City Story, it was heavily implied that Joker blew up a school house of children and that was the moment that Jason decided Joker needed to die. Jason turned off his com link and tracer and tracked Joker to an underground abandoned wing of Arkham Asylum. As Jason entered into the area, he was instantly trapped by Joker's Henchmen. Jason put up a fight but was overwhelmed, beaten, and knocked unconscious. Jason was held captive by Joker who kept him alive, and tortured him both physically and psychologically through the many "games" that he came up with. Those included showing Jason a photo of Tim Drake as Robin six months later, beating him with a crowbar, and branding his face with a branding iron shaped like the letter "J."
Joker continued to do inhumane things to Jason for over a year, and during this time, Batman continued desperately searching for him but with no success, he eventually gave up. Meanwhile, Joker was successful in breaking Jason, and finally convinced him that Batman was the enemy. When Joker recorded an interview with Jason showing that he had broken him (with Todd going so far as to almost reveal Batman's secret identity), Joker then shot him in the chest and stated that he didn't like "tattletales". The video was sent to Batman who presumed that Jason was dead after he saw him get shot.
Joker, thinking that Jason was dead, looked at his body, mocked him, and then left Jason's body to rot, never to return. Jason survived the shot because his body armor protected him from the bullet. Jason regained consciousness, badly wounded, and snapped. After he believed that Batman would never kill the Joker willingly, despite the fact that he had murdered too many people, and feeling both abandoned and replaced with another Robin by Batman, Jason broke psychologically. All he could think from that point was that Batman had to die.
Jason was in the asylum before Deathstroke helped him with his escape. Jason continued searching for a way to truly kill Batman. Jason then assembled his own private militia made up of soldiers, drone tanks, and helicopters. Jason trained them, taught them Batman's tricks, and how to adapt. He even trained some in martial arts and the weaknesses of Batman's Batsuit.
The Arkham Knight was able to monitor the actions of Batman and everyone else involved with Arkham City. During the incident, it was revealed that the Knight had almost entered Arkham City during Protocol 10 to pull Batman out of the rubble, but backed off after seeing Catwoman approach to save him. The Knight revealed that he was going to save Batman and help him stop Protocol 10, as he wanted to be the one to kill him.
Days after the Joker's death, the Arkham Knight witnessed a brief encounter between Batman and the new Electrocutioner, who was quickly defeated. When Batman left, the Knight executed the would-be super criminal with a shotgun blast to the head. Later, in an audio tape, Joker announced his plans to blow up the walls of Arkham City and free all the trapped inmates. The Arkham Knight followed Batman to Arkham City to defuse the bombs. Batman investigated but couldn't find any bombs, he then realized that it was a ruse: Joker's real plan was to blow a big hole into the sewers allowing all inmates to escape through there. Batman found the bomb ready for pick up. Batman defeated the thugs guarding it as the Arkham Knight watched. Batman then tried to defuse the bomb, but it then started counting down from 5, detonated, and destroyed the building. Batman had triggered the final trap that his dead nemesis had left for him. Batman and the Knight survived. Jason located Batman in the rubble and wanting to kill Batman himself, attempted to save Batman, and risked exposure. Before Batman emerged from the rubble, the Knight discovered the remains of the Joker's Diary. Deciding that it would be useful to hold onto, he fled the scene before the emergency crews arrived. Batman remained unaware of his stalker's presence. A family later spotted him in an alleyway after setting fire to a pair of the Joker's former henchmen. When Tweedledie, Tweedledum, and Tweedledee were prevented from stealing Batman's new Batmobile and put in a holding facility, the Arkham Knight shot all three of them dead as well.
The Arkham Knight eventually found Scarecrow after he killed his men as a demonstration of his skills, advised him on how to take revenge on Batman, stated that he had access to an army trained in Batman's techniques, and knew exactly what he feared. It was the Arkham Knight who specifically gave Scarecrow the plan to drive the citizens of Gotham out in order to bring the full forces of his army into the city unchecked and where exactly that Scarecrow could do the most damage. He also devised methods of testing Batman to the limit by committing a force of unmanned drones as he knew if they were manned and armed, Batman would hold back.
The Arkham Knight also wished to kill Batman for causing him great pain and suffering in the past. However, he wanted to kill Batman when he was at his most focused and giving it all to defeat him, hence devising methods which would force Batman to push himself further than ever before telling Scarecrow to put the word out to the Rogues' Gallery that if the plan was followed to the letter, then one of the villains would successfully able to bring down Batman after "pulling him in so many directions". This was shown in the death scenes where the Knight was disappointed with Batman's efforts or being very personal by pulling him towards his face and telling him to look him in the eye when he died. He also relayed some information to Scarecrow that regarded Batman's allies, namely the existence and identity of Oracle. He also accompanied Scarecrow when the latter gathered the top crime lords to call in a truce to eliminate Batman, although he took offense at Two-Face's request for information on how to kill Batman due to viewing him as his prey. This, as well as his overall nature, also made him disliked by the crime lords, especially Penguin, although Two-Face insisted that they gave him a chance due to the Coin telling them to do so. At some point, the Arkham Knight confronted several of Penguin's Henchmen at the docks and murdered all but one of them, broke his arm, and proceeding to threaten him with a gun to the mouth, which the surviving thug then relayed to Penguin, which caused the latter to break his other arm in anger.
The Arkham Knight also reached out and hired those who had various reasons for wanting to kill Batman such as the highly skilled assassin and super-soldier, Deathstroke, to be a back up commander and consultant for the militia forces, and told the one-eyed assassin that he was the toughest combatant that Batman had ever gone up against, and who knew full well the reputation that Deathstroke had for hunting down and killing fellow warriors whom he respected.
The Arkham Knight first made himself known to Batman at Ace Chemicals while Batman was searching for Scarecrow there and found the hostages. The Knight used an attack chopper to provide protection while Scarecrow finished perfecting his new toxin. The Knight abandoned it temporarily to face off with the Dark Knight while he was attempting to rescue the trapped workers, and instructed his men to aim at the less armored sections of Batman's Batsuit. As Batman attempted to leave with the last worker, the Knight forced a showdown between his chopper and the new Batmobile. Batman managed to overcome them and located Scarecrow who managed to escape from his clutches.
The Knight kidnapped Oracle and brought her to Scarecrow as leverage in his plan to destroy Batman. Batman attempted to track Oracle, but the Arkham Knight outsmarted him and shot and injured him in his side by using a weak spot in his armor. Later, the Arkham Knight extracted the Cloudburst from Simon Stagg's Airship after Batman snuck aboard and dealt with both Stagg and Scarecrow.
After taking the Cloudburst, the Knight ordered his army to retrieve Mr. Freeze's wife at the freighter, Nora. If he failed, the Knight had other people to lead the campaign.
The Arkham Knight then activated the Cloudburst, and flooded the city with Fear Toxin. Working together with Poison Ivy, Batman located and freed two ancient trees that could neutralize the toxin, which allowed Ivy to heal and empower them. Batman then destroyed the Cloudburst, which allowed Ivy and the trees to neutralize the toxin, thus saving Gotham from the Cloudburst, but lost her life. The Arkham Knight tried one more time to kill Batman with his personal excavator, and managed to trap him underground in the Batmobile. Batman was able to escape once again, but not before the Arkham Knight had admitted his role in revealing Oracle's location to Scarecrow. The Batmobile, on the other hand, was destroyed.
The Arkham Knight finally revealed his true identity when Batman attempted to rescue Commissioner Gordon from the HQ of the militia at a construction site. The Knight raised his helmet to show his face with the brand, "J" that Joker had left on him during his torture. Jason revealed that during the torture, Joker had convinced him to hate Batman for giving up and abandoning him only after six months while Joker was showing him "tough love" and that it was Batman whom he should truly hate. Jason confronted Batman using his actual name, and demanded that he tell him how long he actually searched before he gave up on him. Batman attempted to explain that he saw the tape that Joker had sent to him of him killing Jason. The Knight bitterly yelled at Batman and told him that the only thing he wanted was Bruce dead, right before he engaged with Batman who managed to hit and damage a part of his helmet.
However, Batman finally managed to get through to Jason after a long and difficult fight. After he finally realized that Batman had never wanted to abandon him, and was willing to help him after apologizing, Jason abandoned his persona and become a violent vigilante who was known as the Red Hood. When Scarecrow trapped Batman at the Arkham Mansion, Jason arrived and freed him.
Some time following Batman's initiation of the Knightfall Protocol, Jason restarted his career as a vigilante. Seeking to eliminate crime lord Black Mask, he attacked his thugs during an arms deal, then proceeded to eliminate his weapon storage unit's security detail. Interrogating a survivor for his location, Red Hood dueled Black Mask in person at his downtown office, proving victorious. However, instead of granting Sionis mercy, Todd kicked the mob boss out of a window to his death several stories below. Alluding to his personal hatred of the Joker, Red Hood told Sionis to greet the clown for him in hell.
In the Legacy of Chris Thorndyke, the Arkham Knight is not Jason Todd, since that Jason is currently the Red Hood, but his identity is still a mystery at this point on, but he does wield a sword and guns and leads a militia and may have connection to Lady Arkham as her knight and he could be a dark reflection of the Batman himself.
And he is reprised by Troy Baker in the English dub and Satoshi Taki in the Japanese dub.