A city under the sea.

it is undefined whom the User is, whether they are an inhabitant of this city or someone who got unlucky and found themselves in this place.

Andrew Ryan: the man who created this city who believes in objectivism. he's a industrialist and believes people should think of themselves rather for others.

Yi Suchong: a researcher and scientist. created a bond pair with these behemoths called Big Daddy and small girls known as Little Sisters. created these gene altering tonics called Plasmids with the help of Brigid Tenenbaum.

Atlas: the man who started the civil war within Rapture. acts friendly with a Irish accent but is actually someone else whom is known as Frank Fontaine. Frank Fontaine: a con man with a criminal mastermind who speaks in a Bronx accent. went through many aliases to avoid getting caught.

Jack: a man who was created inside the walls of Rapture. conditioned to follow orders by uttering the phrase "Would you kindly" but with the help of Tenenbaum they got rid of this brainwashing phrase. uses plasmids and weapons to escape Rapture. doesn't speak and knows only actions.

Brigid Tenenbaum: a woman who discovered ADAM and helped using it as commercial sale. responsible for created Little Sisters and has developed an emotional attachment to them and sometimes refers to Little Sisters as her Little Ones. was born in a place called Belorussia.

Bid Daddy: walking diving suits who protect Little Sisters with their life. a man was inserted and melded inside the suit. can't speak and can only communicate through sounds that are like a whale. variants of the Big Daddy, some that use a drill to attack while others will use a rivet gun or rocket launcher. normally docile until attack or someone gets near their Little Sister.

Little Sister: little girls who have been brainwashed by Yi Suchong into collecting ADAM from the dead. sees the world as some fantastical place and refers to the dead as "Angels" and trusts their Big Daddy and often referring to them as Mr. B or Mister Bubbles.

Splicer: drugged-out junkie high on ADAM. uses melee, ranged and even Plasmids to attack. people so far gone that they barely remember their past life. used to be regular residence of Rapture but got addicted to ADAM and have gone mad.

ADAM: a red substance that gives one the power to alter their genes and give them super-human abilities. EVE: a blue liquid that allows the use of Plasmids. Plasmids: super-human powers that require EVE as fuel to be used.

(Do note: these descriptions are probably not the most accurate to the videogame)