You will encounter the Little Sisters who will help you through the game, if you let them. Decide which route you want to take and stick to it. Saving every Little Sister will be more rewarding but Harvesting will gain you more ADAM.
If you decide to Harvest the sisters you will miss out on the Savior
and Master Gatherer
achievements. You will be given three options with the Little Sisters you encounter, Adopt, Harvest and Save. Adopting will allow you to harvest bodies for ADAM which will go towards Master Gatherer
. Once you are finished gathering with your current Little Sister, take her to a vent and save her. You can gather two bodies with one Little Sister. You won't be rewarded so well in the beginning, but using a Little Sister effectively will reward you in the long run.
You can keep track of how many Little Sisters you need to save by pressing
, and viewing the stats at the bottom of the pause menu.
0 - Adonis Luxury Resort
0 - Atlantic Express Depot
1 - Ryan Amusements
2 - Pauper's Drop
3 - Siren Alley
3 - Dionysus Park
3 - Fontaine Futuristics
0 - Persephone
0 - Inner Persephone
Do not leave an area and progreed to the next without gathering and dealing with the sisters. If you do so you won't be able to return. Gathering does not count towards dealing with each sister. You must either Save or Harvest all the sisters for this achievement.