Horatio isn't exactly a guide or a mentor. He is, however, Hamlet's only real friend and a constant source of advice and support. He helps Hamlet in his attempts to out Claudius as a murderer, and he's the guy left at the end to tell the story to the whole world, or at least to Fortinbras. Oh, and he's also pretty much the only consistently likable person in the play. Yay, Horatio.