In their earlier childhood, "Hänsel" and "Gretel" are forced to participate in pedophiliac snuff films, in which they are either to murder other children or be raped. Such horrific incidents and witnessing continual beatings of other children led them to turn into sadistic, deranged killers. They have a firm belief that their life spans are extended by killing others. This belief probably derives from the fact that they had to kill other children to avoid beatings in their orphanage years.
Unknown to most people, "Hänsel" and "Gretel" are actually two personalities alternately adopted by the two children, both of whom suffer from 'dissociative identity disorder', meaning they swap their identities as "Hänsel" and "Gretel" with each other from time to time (these were their stage names in the films as confirmed by Balalaika). This appears to be indicated through the swapping of clothes and a wig. There are slight implications that they are incestuous, possibly due to the abuse they suffered in the state-run orphanage. It is never made clear of what sex each twin actually is, although they are likely monozygotic twins considering how identical they are so it is likely that they are of the same sex.
In combat, "Hänsel" wields a sharp, wicked-looking battle ax while "Gretel" uses a M-1918 Browning Automatic Rifle that appears to be taller than 'she' is. Both also carry additional sidearms. While they were mentally deranged, were also cunning, having used one of the local orphans as a decoy to distract the bounty hunters and using money to distract Eda. They were also not above using their innocent appearance to deceive their enemies.
As revenge for killing Balalaika's men, Balalaika used herself to lure Hänsel into one of Roanapuar's parks, where he is shot from a distance by one of her snipers in the leg and the arm. Unsatisfied, Balalaika decides to not torture him but to watch him bleed to death, and witness him crying in his final moments, much to Boris' displeasure. Meanwhile, Gretel manages to escape and eventually boards the Black Lagoon of her own volition. On the ship, she and Rock bond, and his kindness prompts her to open up and reveal her and Hansel's horrific backstory, as well as their twisted belief system and that Gretel is Hansel wearing a wig. Rock, completely distraught by hearing this, desperately hugs Gretel/Hansel, who seems genuinely shocked at experiencing the honest affection of another human being. As Rock hugs her and tries to change her view of the world, Gretel momentarily looks affected by his earnestness, and becomes saddened that she upset him. Attempting to cheer him up, she shows him her genitals. Now completely horrified, Rock leaves the cabin, and Revy, thinking the girl was simply messing with Rock, punches her in the face. This brings Gretel's fierce and sadistic front back up, and she taunts Revy about how they are the same. However, she remains affected by Rock's kindness and takes a genuine liking to him. The two arrange to meet up and have a picnic at some point in the future.
However, just as Gretel disembarks she is gunned down by Elroy, the get away man Dutch contacted, who was bribed by Hotel Moscow. While Rock is initially horrified at her death, he expresses relief that Gretel and her brother are now at peace. After she had died Dutch suggested that they bring something to cover her up however, Rock refused, saying that it would be better if she had died looking up at the sky.
- The characters' names are taken from the German fairy tale, '. Many of the series' characters compare them to a similar pair of disturbed, sadistic twins from the American film The Shining. In the manga, the song sung by Gretel to Rock on the Black Lagoon is called "Midnight, the Stars, and You", the song used in the ending credits to The Shining. In the anime, she sings "The World of Midnight", made for the anime and sung by Minako Obata. In the TV Show, their specific genders are unclear, and in Omake 4, it makes it seem like nothing changed because the children's true individual genders are unknown.
- Instead of the ending with Revy walking along the shoreline it is Gretel and Hansel, who walks the shoreline first only Gretel is shown walking barefoot in basically the same manner that Revy is in the outro only later to shown that she walking beside her brother Hansel, then Ending with them both watching the sunrise.
- Gretel is a vert talented singer, as shown when she sings for Rock, causing Dutch to remark "Girl sure can sing. Hard to believe it's the voice of a homicidal maniac. A devil with the voice of an angel."
- The horrific reaction of Rock when Gretel lifted up her skirt was likely partly due to her past when she was forced to take part in pedophiliac activities and snuff films, which most probably hideously disfigured her genitals, and partly the fact that Gretel thought that was an appropriate way to thank him for his kindness and try to cheer him up.