Big Sister & Little Sister
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.