Little Sisters are genetically modified girls who were trained to produce and process ADAM. Their bodies were induced by Sea Slugs which give them virtual immortality. Despite even that, they are quite defenseless against any greedy Splicer. That's why they need Big Daddies to watch over them. These powerful bodyguards live and breathe only to ensure that Little Sisters are safe.
The girls are the only source of ADAM in the game. And what do we need ADAM for? It's a currency that we use to buy genetic upgrades in machines called Gatherer's Gardens. The more ADAM we have, the easier the game gets: we can afford buying more health, more EVE, more plasmids, tonics, more slots. Hence the dilemma: should we harvest Little Sisters for Sea Slugs, killing them in the process (160 ADAM each), or rescue the girls for a lot less cash (80 ADAM each)? The choice is yours, really - but it determines the ending you'll see in the game. There are three possible endings:
- Good: you saved all the girls and in the end decide to rescue them from the underwater city, give them a new life on the surface.
- Bad: you saved some girls, but also killed some of them; you decide to take over the city and create your own army of Splicers to take over the world.
- Very Bad: you killed all the girls; the same as bad, the only difference is the narrator's tone of voice (a little more angry, so to say).
HINT: If you decide to rescue the girls, each time you save three of them Tenenbaum will send you a gift. It will wait for you inside a teddy bear in the nearest Gatherer's Garden. It's the only way to get some types of plasmids. Additionally, there's 200 ADAM in each reward.
To get the ADAM - one way or another - we have to dispose of the Big Daddy first. There's no real state-of-the-art way to do this, at least not until late in the game. Big Daddies are the toughest opponents in the game. Generally: you should always use anti-armor ammo and electricity. At the beginning of the game you may want to accumulate some Electric Bucks for the shotgun especially for Big Daddy fights.
The best weapon against Big Daddies is the Chemical Thrower with Electric Gel ammo. The ammo itself is difficult to come by, but results are more than satisfactory. A little more common way to eliminate Big Daddies is to use "classical" heavyweight arms, such as grenades, proximity mines and other types of explosive ammo. Also, forget about using pistol (even with anti-armor ammo) and wrench. It's just a waste of time.
There are two types of Big Daddies in the game: Bouncers and Rosies, each in two versions: standard and elite. Altogether they use 5 different attacks: melee punching, throwing mines, shooting their guns, stunning, and charging.