Mister Hammer

Mister Hammer, also referred as Mr. Abramovici, is seen as Joker's one armed lieutenant in his army. After Joker's death, he later joined Black Mask with other Joker thugs. He was mentioned to be with his conjoined twin, Sickle, performing at a Soviet Union circus. Yet, they got in a huge argument and broke up, literally. They moved to Gotham for some reason, and became criminals.

Batman: Arkham City
When Batman goes in Joker's Funland and tries to find Joker, who remotely tried to assassinate Catwoman, he goes in the vents and tries to surprise the Joker, but he comes into the middle of Harley's announcement. She explains to her group that Dr. Stacy Baker failed Mr. J and that she couldn't cure him. Baker pleads that she couldn't do anything but Harley calls her a quack, and then kicks her from Joker's office onto the floor below. Harley announces "Do you want to see a show?" and they start to chant "Kill," as Mister Hammer tries to kill her with his sledgehammer, but she starts to crawl away. As soon as Mister Hammer got her cornered, Harley stops him by saying that the Joker needs her alive, so she can get tortured more. The thugs dragged her away, as Batman follows them, going through a bunch of thugs.

After saving Stacy Baker and getting the Remote Electric Charge, he makes his way to Joker's office, where Harley locks the doors from him getting in. Yet, by using the REC, he knocks the steel doors, blocking him from the Joker. Harley yells at him for knocking the steel doors, but as soon as he grapples up to the office, Mister Hammer kicks back down to the floor. Harley says, "Meet Mister Hammer. He going to teach you a few manners." After fighting Mister Hammer and a bunch of thugs, he wins and makes his way towards Joker, yet he finds him dead and Harley crying. However, it was just a gag and Harley knocks out the Bat, with her bat.

When Batman wakes up, he has been infected with Joker's blood, and must find a cure for both himself and the Joker. He has to go meet Mr. Freeze to get the cure, but The Penguin kidnapped him. After defeating the Penguin, Mr. Freeze needs immortal blood to make the cure work, and Batman remembers about Ra's al Ghul, yet as one of the captured people of Penguin's hears this, the assassin escapes. Batman tracks the assassin, meets Talia and the League of Assassins, and fights Ra's for his blood. Getting back to Mr. Freeze, Mr. Freeze breaks the vial and says he wouldn't save the clown, since he kidnapped his wife, or Batman. After defeating Mr. Freeze, Harley sneaks in to get the cure.

Going back to Joker's Funland, Harley is tied up for some reason inside the funland. Joker is there, putting up his make-up, and battles Batman. Thinking it's over, Batman sees that Venom Henchman, Mister Hammer, and Joker's thugs come to help Joker. After defeating all of them again, the building collapse on Batman. Batman sees that Talia and Joker are talking about Joker leading the League of Assassins, and he accepts. Batman is knocked out, but Catwoman comes to save him. Yet, Batman deals with both Hugo and Ra's for starting Protocol 10. Ra's sponsored Hugo and Sharp, and together, they started Arkham City, and started all of this to happen. Both men die as Wonder Tower explodes, and Batman finally goes to Joker.

Being in Monarch Theatre, after finding out that someone been impersonating Joker, Talia is shot by the Joker and dies. Joker laughs and claps,and announces that the impersonator is Clayface. After defeating Clayface, Joker comes up behind Batman and stabs him, which cause Batman to drop and destroy the cure. Laughing for the last time, Joker dies and Batman brings his corpse out to the GCPD.

After Joker's death, Robin goes to find Roman Sionis, as he becomes a new gang leader in Arkham City. He goes through Black Mask's hideout, where he finds out that some of the Penguin's thugs joined him, and that Riddler is also helping Black Mask. Lastly, he goes into the Freight Train and beats through some of the Joker's thugs, that have also joined him. After getting through three cars, Robin comes to end to find few Joker's thugs, Mister Hammer, and Black Mask blocking the control box. After beating the Joker's thugs and Mister Hammer, he leaves Black Mask for last, apprehends him, and destroys the control box that's moving the train.

Trivia

 * Mister Hammer has a tattoo that tells everybody that he's from Russia, and his arm gut cut off is explained when the player goes to a group of people talking about how Mister Hammer was a conjoined twin, and how after an argument, they split up, going to different groups.
 * Mister Hammer goes with Joker, while Sickle goes with Penguin.