Physical: The two twins are so identical that they are given one name and cannot function without each other:

They breathed together, they grinned together, they were chunky and vital. They raised wet lips at Ralph, for they seemed provided with not quite enough skin, so that their profiles were blurred and their mouths pulled open.

Involvement: Samneric are often in charge of watching the signal fire. They too side with Ralph, but are captured in the end by Jack. Their loyalty doesn't extend as far it should; they eventually confess to Jack where Ralph is hiding before he his hunted.