Little Sisters are the product of a scheme concocted by Bridgette Tenenbaum, Dr. Yi Suchong, and Frank Fontaine in order to harvest mass amounts of Adam. After a rare sea slug was found to contain the substance, Tenenbaum came up with a method to implant the sea slugs into young female children in order to increase the amount of Adam produced. Suchong was responsible for brainwashing the children into extracting additional Adam from dead bodies and for pairing them up with their Big Daddy protectors. And, finally, Frank Fontaine founded an orphanage as a home for all of the Little Sisters they recruited.

Little Sisters travel through Rapture using the many vents that snake through its walls (or, as Suchong called them, "hidey-holes"). While they're virtually immune to physical damage because of the Adam coursing through their veins, Little Sisters are still weak children that stand the chance of being exploited by the crazed Splicers running amok in Rapture. Therefore, they won't leave the protection of their "hidey-hole" without a Big Daddy present to protect them. Every time you encounter a Little Sister in the underwater city, expect her to be accompanied by one of two versions of Big Daddy.

Should you slay her Big Daddy protector, you'll have the option to either harvest or rescue the Little Sister. Harvesting the sea slug from inside her takes the young girl's life, but nets you 160 Adam. Rescuing her with Tenenbaum's plasmid removes the neurological condition instilled by Suchong but only nets you 80 Adam. For every three Little Sisters you rescue, though, Tenenbaum will send a gift of Adam and other goodies to the nearest Gatherer's Garden vending machine.

There are five research levels that can be attained by snapping photos of Little Sisters:

• Research Level 1: Small increase to your maximum health and EVE.
• Research Level 2: Small increase to your maximum health and EVE.
• Research Level 3: Small increase to your maximum health and EVE.
• Research Level 4: Small increase to your maximum health and EVE.
• Research Level 5: Small increase to your maximum health and EVE.