When they first meet, Ralph seems to have little respect for Piggy--not returning the courtesy of asking him his name, telling the others about how Piggy was called that name back home, etc. Regardless of how Piggy was perceived by others, though, Ralph still remains friends with him.
As time progresses, Ralph realizes the true friend that Piggy is. Not only is Piggy insightful about what needs to be done, he is also a loyal companion even as their society begins to fall apart on the island. Ralph is forced to protect and stick up for Piggy, such as in the scene where Ralph and Piggy confront Jack and his "tribe" in order to get back Piggy's glasses.
At the end of the novel, Golding makes it clear how important Piggy was in Ralph's life, as Ralph, once rescued, wept for the "one true friend" that died on the island.