Ralph and Piggy complement each other and strengthen the leadership which is important in keeping Jack under some form of control. Golding uses the two characters to reveal
the struggle between reason and emotion
There is however, a reliance on each other and when Piggy dies
Ralph the daydreamer struggles to remember the rational ideas Piggy told him about rescue
He has become so used to Piggy's advice that he is unable to form his own opinion without guidance.
Piggy is reliant on Ralph to keep him safe from Jack and his tribe. Piggy is scared of Jack and recognizes Jack's tendencies and savagery. He knows that his own preservation depends on Ralph's position as leader.
If you give up," said Piggy, in an appalled whisper, "what 'ud happen to me?"
Piggy fulfills a supportive role to Ralph's charismatic personality and the one without the other is doomed to fail.
Ralph feels responsible for Piggy's death as it was his responsibility to protect him and, as leader, he represents
order, authority, dialogue, democracy
so, at the end, he accepts responsibility for everything that has happened because, amongst other things, he failed Piggy.