The twins in are named Sam and Eric, but are always called Sam ’n Eric.
The twins do not seem to have separate identities of their own. They are always together, in fact to the extent that instead of being referred to by separate names, their names run together into Sam ’n Eric or sometimes Samneric.
They were twins, and the eye was shocked and incredulous at such cheery duplication. They breathed together, they grinned together, they were chunky and vital. (Ch. 1)
The twins are part of the group of older boys on the island, and are active in decision-making and committee meetings. They are not easily pushed around, and usually wait for Ralph to make decisions and support him. They stick with Ralph to the end.
Like most twins, Sam and Eric often finish each other’s sentences. One of their most significant events is when they are watching the signal fire one night and fall asleep. They see the parachutist and think it is the beast.
“Let’s warm up.”
“We’ll only have to fetch more wood.”
“—dark. All right, then.” (Ch. 6)
The twins run down to tell Ralph what they saw, convinced it was the beast in a combination of their sleepiness and the fear from being alone on the mountain in the dark. The twins are the first ones that see something that actually is real, even though it is not an actual beast.
The twins are also important because they stay loyal to Ralph. Piggy is upset when they go to Jack's party, but Ralph understands why they went.
“That’s where they’ve gone. Jack’s party.”
“Let them go,” said Ralph, uneasily, “I don’t care.”
“Just for some meat—”
“And for hunting,” said Ralph, wisely, “and for pretending to be a tribe, and putting on war-paint.” (Ch. 9)
However, when they go back to try to talk sense into Jack, it is a disaster and Samneric are captured. Ralph seems helpless to protect them.
Samneric protested out of the heart of civilization.
“Oh, I say!”
Their spears were taken from them. (Ch. 11)
Of course, Samneric get off easily if you consider what happens to Piggy, since he is killed, but it really is interesting how they react when they are captured.
Sam and Eric, though treated as one entity and one character, are loyal to Ralph to the end. Ralph represents civilization, while Jack represents savagery. We know which side these two are on.