[Discussing stories in detail inevitably involves some spoilers.
The ones in this post, however, should be fairly mild.]

The wise old mentor is a staple, not only in fantasy, but in all kinds of stories.  From a narrative point of view, though, these mentor figures are rather an inconvenience – which is why they so frequently go missing.

Gandalf the Grey, the very archetype of the mentor in an adventuring party, is kept offstage by other engagements for much of The Hobbit.  In The Lord of the Rings Tolkien goes so far as to have him perish (not permanently, to be sure).

In the Harry Potter stories, Albus Dumbledore tends to be curiously inactive – he’s not around when the crises occur – though this changes over the course of the series, until he’s fully engaged toward the end.

Professor X, of the X-Men, is generally confined to a wheelchair, which keeps him out of the action.  In the first couple of X-Men movies, he’s also hors de combat much of the time.

Gordon Ashe, the main character’s mentor in Andre Norton’s Time Traders­ series, often happens to be sick or injured.

And of course Obi-Wan Kenobi dies about a third of the way through Star Wars:  A New Hope – even if he keeps popping up periodically through the three original episodes as a Force ghost.

Why does a writer introduce these characters, only to shuffle them offstage as soon as possible?  Consider what the mentor contributes:

1. Power.  The mentor is often a fully-developed version of what the hero is becoming, as in Star Wars.  If not, like Gandalf, he is typically a powerful figure in his own right.
2. Knowledge.  Gandalf knows how to terminate trolls and how to open the doors of Moria (Frodo helps in the movie, but not the book).  Mr. Miyagi in The Karate Kid (1984) know how to train in karate.  Obi-Wan knows where to find Yoda.
3. Wisdom.  The mentor often advises the hero about life – not specific information, but how to live in a more global sense.  “Do, or do not; there is no try.”  “All we have to decide is what to do with the time that is given us.”  “Man who catch fly with chopstick accomplish anything.”

Here’s where the problems start to arise.  If the mentor is a powerful figure, why isn’t the mentor out fighting the battle, rather than sending the hapless (hero) apprentice?  The more impressive the mentor’s abilities, the harder it is to avoid having the mentor displace the hero.  With the mentor in action, things would be too easy for the main characters.  (There’s a parallel problem in children’s adventure stories – how to get the children away from parents and other caregivers so they have to act on their own.)

Knowledge poses a lesser problem, but a good storyteller still rations the mentor’s advice closely.  It makes for better drama if the hero doesn’t quite know what to do and isn’t fully trained.  (The tagline for the first World of Warcraft game expansion was:  “YOU ARE NOT PREPARED!”)  Luke Skywalker is more thrilling as a brash but vulnerable neophyte facing Darth Vader than he would have been as a fully seasoned Jedi knight.  The writer may prefer to have the hero not fully informed – if only to enable a shocking surprise at the right moment.

The problems are not as severe with the mentor’s third role, as dispenser of wisdom – though it still falls to the hero to implement the teacher’s wise counsel, when the crisis comes.

Authors thus expend a lot of effort to keep mentors out of the action, leaving the heroes on their own to apply what they have learned – or fail to do so.

Gandalf dies in Moria; he returns, but by that time he’s cut off from Frodo and Sam, who most need his guidance.  (“Its name was Cirith Ungol . . . Aragorn could perhaps have told them that name and its significance; Gandalf would have warned them.”  The Two Towers, ch. IV.3)  Gandalf is present, however, for the big battle scenes, and is ready to take on the Witch-King at Minas Tirith.  In effect, Tolkien has held Gandalf’s might in reserve:  as the enemies get bigger and worse over the course of the story, it makes sense to bring the powerful mentor back in, to even the scales.  We see the same kind of progression in Harry Potter, where Dumbledore takes a more direct hand as the story goes on (though he’s removed to make the final battle more challenging).

In the Silver Age comics, the wheelchair was enough to keep Professor Xavier out of the action most of the time.  In the movies, his range and power is vastly expanded, and he has to be rendered comatose to keep him out of the fray.

George Lucas managed to eat his cake and still have it.  He opts for the drastic solution by killing off Obi-Wan for good.  But Obi-Wan’s continuation as a ghost allows him to keep providing occasional advice – not to mention retconned explanations (“From a certain point of view”).

E.E. Smith’s classic Lensman series gives us an entire species, the Arisians, as mentors.  One character, a “fusion” of four Arisians, is actually known as Mentor.  Smith crafts his story to produce fairly subtle and plot-central reasons for keeping the Arisians out of the main conflicts.  At first they need to conceal their existence from their Eddorian adversaries.  Later, they need to keep their vast powers under wraps so as not to undermine the confidence and self-reliance of the Galactic Patrol.  But the Arisians do emerge in time for the climactic battle – which could not be won without both the Arisians and the Patrol (and the Children of the Lens, but that’s another story).

The mentor isn’t always missing in action.  A writer can engage the mentor figure in the story, if proper caution is employed to dodge the above problems.  For example, the social conditions of The Karate Kid mean that Mr. Miyagi can’t simply obliterate the adversaries.  He has to equip Daniel to fight a duel, in which third parties aren’t allowed to intervene.

Another way of handling it is to have the hero and mentor fighting on separate tracks.  Thus, in The Mask of Zorro (1998), the older Zorro is supposed to be dead and has to stay in disguise for most of the story.  But during the climax he is revealed and takes on his old nemesis, while the new Zorro is saving lives and fighting his own opposite number.

One of the reasons the absent mentor appeals to us, I think, is that it reflects something we experience in real life.  As we grow older, we do leave our mentors behind.  Generally, we outlive them – and sometimes feel inadequate without the advice and assistance of those who seemed towering figures in our youth.  Yet, just as in a story, this is necessary if we are to grow up.  In the end we succeed our mentors, and become the heroes of our own stories — and, in turn, mentors to the next generation.