Once genetically conditioned in the research labs of Point Prometheus and Fontaine Futuristics, a Big Daddy's main purpose is to wander Rapture, searching for a Little Sister to protect, while she is gathering ADAM. Once paired with a Little Sister, the Big Daddy follows her like a faithful dog and fiercely fights off anyone who approaches her. A standard Big Daddy has extremely limited mental abilities and no free will. They exist as docile "sleepwalkers" who will fight to the death only if their Little Sister is threatened.
The Big Sisters are Little Sisters who have passed into puberty (or beyond). Overexposure to ADAM throughout their lives has had major effects on their mental stability and caused them to be extremely violent. The ADAM also grants them immense strength, agility and the use of powerful Plasmids. They have the appearance of thin, feminine versions of Big Daddies, possessing very similar looking diving suits and equipment. They also have cages on their backs which are used to carry Little Sisters. When the player incurs a Big Sister's wrath, the red porthole on the front of her helmet will glow, and then the Big Sister will launch her attack. Each Big Sister has a large ADAM needle on her left arm which is used for precision melee attacks and to drain ADAM from live victims, allowing her to heal. Big Sisters are able to jump across an entire room, have immense strength and agility, and possess powerful Plasmids, making them formidable enemies.
Little Sisters continue to wander the halls of Rapture, extracting blood from corpses, and processing it through their bodies to be purified into ADAM. Many of the Little Sisters in the city were taken from the surface, as revealed through the course of the Something in the Sea website. Little Sisters are protected by Big Daddies, and Subject Delta has the ability to "Adopt" these Little Sisters to collect ADAM from corpses. Delta can also "Harvest" or "Rescue" them. When Subject Delta deals with a certain number of Little Sisters, a Big Sister will come to attack him for his interference.
"Splicer" is a term for the genetically spliced citizens of Rapture, who became aggressive lunatics after their minds and bodies have been distorted by unchecked genetic manipulation through the use of ADAM. Once addicted to ADAM, a Splicer's only concern is procuring more. They see anyone unfamiliar as a potential score, and will attack anyone for a chance to gain a little ADAM.
Hostile Security Bots, Security Cameras, and Turrets will target the player on sight. In BioShock 2 there is a new type of Turret. It is a smaller, portable kind that can be carried around (See Miniature Turret). These are also used as weapons by the Rumbler Big Daddy.