Bridgette Tenenbaum
Dr. Bridgette Tenenbaum is a German genetic scientist who helped originally develop ADAM. She is also the "mother" of the Little Sisters. She created them and cares a great deal about their safety. You are able to win her allegiance by saving the Little Sisters instead of harvesting them for ADAM.
Tenenbaum managed to survive being held prisoner in WW2 where she discovered she had a love for science. The Germans picked up on this and she was a doctor for them at the age of 16. Tenenbaum would notice that what some of the doctors were doing was wrong and correct them. She was a scientific genius at a very young age.
Tenenbaum was the first to discover ADAM. Tenenbaum discovered a slug that changes the body and the mind giving the user many powers. At first she was snubbed by the scientists of Rapture but with the help of Frank Fontaine she was able to get her invention off the ground and quickly became one of the most well known women in Rapture.
With Fontaine's help she was able to create the Little Sisters, but once she realised what she was doing she became disgusted with herself. Realising that she was simply creating monsters, Tenenbaum began to hate herself for what she had done. By the time the player arrives in Rapture she has seen the error of her ways and begs the player to help the Little Sisters. If you do what she asks you only retrieve 80 ADAM per rescue, but after every three rescues she rewards the player with 200 ADAM and various other items including rare Plasmids. If you choose to harvest the Little Sisters, you retrieve 160 ADAM, and she does nothing but express her anger towards you. The choice is left to the player.
Towards the conclusion of the game, Tenenbaum helps the player by removing Fontaine's mental conditioning and showing him what he really is. She frees him and lets him live a real life. In the ending of the game where Jack raises the Little Sisters, it is likely that Tenenbaum either came with Jack and continued to be a motherly figure, or with Andrew, Fontaine, and most Splicers dead, she stayed behind to look for Little Sisters to exorcise and/or protect.