Mister Hammer

Mr. Abramovici, due to his signature weapon called Mister Hammer, is a former Russian circus performer who works for the Joker after having been separated from his twin brother Sickle who then worked for the Penguin in Batman: Arkham City.

After the main story ended, both Hammer and Sickle have forgiven each other, and mutually wished to be reconnected with each other.

Biography
Mister Hammer, also referred as Mr. Abramovici, is seen as the Joker's one armed lieutenant in his army. He was mentioned to be with his conjoined twin, Sickle, performing at a Russian circus. However, they were forcibly split up by Joker as he believed they would function better as fighters separately rather as one force. Sickle was then violently thrown out of Joker's gang as the Clown Prince of Crime believed his brother was more than enough for his army. Angry at his brother for remaining at Joker's side, Sickle ends up joining The Penguin, Joker's rival, and his gang in retaliation, hoping to show his former employer and brother just who is the strongest Abramovici; thus starts a true brotherly rivalry.

Mister Hammer and his brother Sickle were conjoined, orphan twins who performed in a Russian circus, developing a reputation for brutality which attracted the attention of the Joker. Joker sent his cohort Harley Quinn to recruit the twins, but the circus manager refused, however he subsequently turned up with a Glasgow smile carved into his face, and the twins were recruited into Joker's gang. During a fight between the Abramovici's and Batman, Joker had an ingenious idea to separate the brothers, and acquired the help of Dr. Thomas Elliot to perform the surgery and cut the twins in two. After the surgery was completed, Joker favourably chose Mr. Hammer to be his "right-hand man" and left Sickle for the Penguin, who hired the twin to be apart of his gang. Ever since, the Abramovici's operated alone in Gotham, and later Arkham City, however Sickle always had a disliking for Joker for separating him from his brother.

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 beat 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 him back down to the floor. Harley says, "Meet Mister Hammer. He's going to teach you some 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.

While Batman traveled beneath the subways of Arkham City to find Ra's al Ghul's blood sample, Hammer led a small group of Joker's men beneath the sewers to control access to the subways, and formed an alliance with Killer Croc, as they both came from a similar past. Croc promised Hammer to only feed on the Penguin and Two-Face's men and to spare Joker's, however during the events of Protocol 10 Croc fed on the bodies of any inmates who were killed during Strange's plan and washed up beneath the sewers.

Later on Batman encounters Hammer again during a fight with Joker at the Steel Mill. After defeating all of them again, the building collapses on Batman, however Hammer did survive the incident.

After the Joker's death, Sickle and Mister Hammer are found together in the Steel Mill, talking about how they wanted to be together again. If you scan them, you will answer a Riddler Challenge. Although they are both enemies, they will both ignore you if you do anything.

Following the brother's reuniting, 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 the Riddler is also helping Black Mask. Lastly, he goes into the Freight Train and beats through some of the Joker's thugs who went rogue after the fall of Joker to join Black Mask. After getting through three cars, Robin comes to end to find a few thugs, Mister Hammer, and Black Mask blocking the control box. After beating the 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 Hammer and Sickle "☭" tattoo, a symbol formerly used by his country, Russia whilst Part of the Soviet Union. It is also fitting for him and his brother, due to their names and weapons respectively.
 * Mister Hammer appears in the "Joker's Carnival" challenge map along with Sickle.
 * It takes 50 hits to take him down. If you do so uninterrupted you can get the 50X achievement by fighting him alone.
 * In the comic, he was shown to be able to go toe-to-toe with Killer Croc. After the draw the two were able to connect due to their backgrounds in the circus.