Character Analysis
The mentor is…well, pretty much the best. Sure, they might not be young n' fit (otherwise they'd go do the Big Heroic Job themselves), but they're usually patient, kind, quick to offer you a nice hot cup of tea and, oh yeah, wise.
If the mentor is already a herald, we get to know their true self pretty quickly… and they're always pretty awesome. Even if the mentor isn't, he or she has some serious mojo to lay down on the hero. The mentor tells the hero about his destiny and helps him awaken the hero within:
Not infrequently, the supernatural helper is masculine in form. In fairy lore it may be some little fellow of the wood, some wizard, hermit, shepherd, or smith, who appears, to supply the amulets and advice that the hero will require. (66.2)
That makes the mentor the hero's first friend (or at least ally) in the adventure that he or she is on. It might not always be an amiable relationship (remember how sketchy Haymich Abernathy is at first?)…but it definitely gets results.
In that sense, the mentor gives the hero a blueprint to follow: a way of looking at the world and responding in the proper (i.e., brave, noble and generally Team Good Guy-ish) way. In some cases, the mentor may even be the hero from a previous adventure, ready now to pass what he or she has learned on to a new generation—we see this play out in a big way in The Force Awakens.
But even if they've never played at being a hero, they're always there when the hero takes those first steps. And hey—even though powerful figures like Gandalf, Yoda, and Mr. Miyagi top the list of Most Famous Mentors, there's always room for mentors whose superpower is basically just being orderly and witty (Albert from Batman), knowing a lot about Walt Whitman (John Keating from Dead Poets Society) or just having the common sense to tell his nephew that "With great power comes great responsibility."