Big Sisters are Rapture's fiercest predators. Despite being covered in armor they possess deadly agility and can strike from anywhere. Charged with protecting Little Sisters, Big Sisters keep an eye on all of the ADAM in Rapture and punish anyone that disrupts the balance. Able to throw objects telekinetic-ally, leap off walls for powerful melee blows, throw fireballs, execute devastating charge attacks and even extract ADAM directly from living Splicers, Big Sisters are the worst nightmare of anyone unlucky enough to draw their ire.

Little Sisters are young girls who have had ADAM-producing slugs implanted into their bodies, enabling them to drink the blood of dead Splicers and extract usable ADAM from it. This makes them incredibly valuable in Rapture, where Splicers crave ADAM enough to kill for it. Because of this they are accompanied at all times by Big Daddies, hulking protectors in diving suits who will defend Little Sisters to the death. Little Sisters are driven mad by their constant exposure to substance ADAM but are not beyond redemption; in the right hands they can find salvation and be turned back into the innocent little girls they once were.

Little Sister has an ADAM syringe and ball jointed neck and shoulders, hinged elbows and knees and a v-crotch. And yes, she does actually fit in Big Sister's basket.