Friday's real name is never revealed. He was a member of a local tribe who Crusoe saved from being eaten on a Friday (thus, the name). He has become a archetype for the "noble savage" mentioned by Rousseau and his name has become synonymous with a faithful servant. The term, "my man Friday" has come to mean a person who serves another faithfully. In contemporary times, many are uncomfortable with Dafoe's depiction of a lowly native who must be taught Christian values. But that's probably one of the reasons his original name is never mentioned. Dafoe is suggesting that It is only after he meets Crusoe that he gains an identity.