Heroes is the plural form of hero, denoting more than one. Example: There were many heroes during Nine-Eleven. Hero's is a possessive form denoting something that belongs to the hero. Example: The...
It could either be considered Jason Grace or Percy Jackson. Jason is lost, figuring that he lost his memory, and doesn't know where he came from. However, at the end of the story they realize that...
The heroes have yet to be revealed, perhaps in book 2 (The Son of Neptune?) Jason, Percy, Annabeth, Piper, and Leo seem to be the first five in the group and Hazel and Frank the other 2.