Hansel and Gretel are twin orphans from Romanian who were raised by violence to become sadistic killers for hire.
Their first appearance was in Black Lagoon Volume 2 CH. 11 "Bloodsport Fairy tale PT.I", and her first anime appearance was in Black Lagoon: The Second Barrage - Episode 13 "The Vampire Twins Comen". The characters in the Japanese series are voiced by Omi Minami(Hansel) and Tomoko Kaneda(Gretel), and the U.S. dub produced by Ocean Group is done by Ashleigh Ball(Hansel) and Jocelyne Loewen(Gretel).
Both appear as children barely into their teens that dress in Gothic clothing and stick to specific weapons to kill others. Hansel favors large axes while Gretel uses a Browning Automatic Rifle exclusively. Due to their abusive childhoods, they only believe in killing as many people as they can. They see it as a means for immortality. As long as they keep killing people they'll live forever.
They got their start when they were abandoned by their parents to an orphanage before falling in the hands of organized crime. At first they took part in child pornography. Then they were put in snuff films where they were forced to kill other children until becoming psychologically broken. Murder, torture, and death became the only source of joy for the twins. Eventually they turned on their captors and escaped out into the world ready to act out their "kill or be killed" beliefs.
They can also trade off playing brother and sister using a long haired wig and switching clothes. To become seemingly interchangeable, they can also mimic each others voices and use each others weapons. By their appearance in the anime and manga, there is no real way to tell which one is Hansel and which one is Gretel.
The twins first appear as new killers in Roanapur, but are later revealed to be brought in by Italian mob boss Verrocchio to kill Balalaika and drive out rival mobs. The plan backfires as Hansel and Gretel kill indiscriminately until brutally murdering two members of Hotel Moscow who also served under Balalaika in Afghanistan. It triggers a manhunt involving most of Roanapur causing Verrocchio to try and kill Hansel and Gretel to cover it up. They get to him first, but still try to go through with his plan only because they "want to."
Hansel is lured into a trap by Hotel Moscow where he tries to confront Balalaika. He bleeds out and dies after sniper teams cripple his leg and shoot off his hand. Gretel tries to escape by hiring Lagoon Company to smuggle her out of the city. She has a brief conversation with Rock who tries to convince her there is more to life than the endless cruelty she experienced in Romania. She tries to compensate his kindness with sexual favor. When she reaches her destination, she is shot in the head and killed by an associate paid off by Hotel Moscow.
Hansel uses a pair of axes to grizzly effect and has enough strength and skill to cut the limbs off people much older than he is. He keeps them hidden in his jacket along with a pair of small revolvers (that are never seen fired).
Gretel’s weapon of choice is a Browning Automatic Rifle with a doll hanging from the bi-pod legs. Despite its size she has no trouble firing it from the hip or using the carrying handle to steady her aim. When not in use, she keeps it wrapped up in a blanket where only the doll can be seen.
Other than their strength, they rely more on their seemingly innocent appearance as children to get the jump on their targets.