Looks like he was Fukubei's sidekick, and also wore a mask like Sadakiyo in his childhood. So nobody remembered him, thinking it was Sadakiyo. Then for some reason, which I forgot, Fukubei spread the rumours of his death, and after that he spent his life in a mask, then as Friend's double.
I wonder if he was Fukubei's brother or just some random looser boy, though. Apparently, after Fukubei's death, he had a plastic operation to look like him.