As a result of being repeatedly forced to participate in pedophilic snuff films in which they were either raped or forced to murder other children, and being repeatedly forced to watch the people running the orphanage beat one of the children to death in front of them, the twins eventually became deranged, sadistic killers just to survive. 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 being "Hänsel" and "Gretel" with each other from time to time (it is likely that these were their screen names in the films). There are slight implications that they could be incestuous, possibly due to the abuse they suffered in the state-run orphanage. It is never made clear of what gender the twins are: in one scene, "Gretel" (formerly "Hänsel" before swapping) shows 'her' genitalia to Rock in what she presumably came to believe was a show of gratitude, causing him to flee in disgust.
Both of them are white haired and dressed in dark gothic clothing, such as "Gretel" is dressed in a Gothic Lolita outfit.
In combat, "Hänsel" wields a sharp, wicked-looking battle axe while "Gretel" uses a M1918 Browning Automatic Rifle that appears to be taller than 'she' is. Both also carry additional sidearms. While they were mentally deranged, Hänsel and Gretel were also cunning, having used two other children as decoys to distract the bounty hunters and using money to distract Eda. They were also not above using their innocent appearance to deceive their enemies.
- The characters' names are taken from the German fairy tale, Hänsel and Gretel. 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.