I'll draw most of my examples using Dumbledore from the Harry Potter books, Yoda from Star Wars, Gandalf from The Lord of the Rings, and the "original" mentor, whose name has morphed into this meaning, Mentor from The Odyssey. Homer's Mentor is the probably the one least well known today, so here's a brief explanation of him (her): Mentor was actually the goddess Athena, disguised as a mortal male, so that she could appear to Telemachus, the son of Odysseus, and give him advice, assistance and encouragement.

Warning: spoilers abound below!

Characteristics

What are the typical characteristics of mentors? As the mentor imparts wisdom, the mentor should be both learned and experienced. Often stories display evidence of learning and experience through objects such as an immense library, experimental equipment, magical devices, and so on.

Usually mentors are significantly older than the hero. Sometimes they are so old that the only way to show him is to use a wizened puppet, such as Yoda in Star Wars. From my readings of the Harry Potter books, Dumbledore's age seems to vary in the earlier volumes (J.K. Rowling admitted in one interview she's not great with math) but in the last volume she fixes it by attaching events in Dumbledore's youth to a certain period.

Mentors are frequently powerful. Both Gandalf and Dumbledore are extremely talented wizards, while Yoda is a Jedi Knight. Mentor of The Odyssey, as he is really Athena, is actually divine.

Mentors often speak cryptically, at least from the perspective of the hero. This may be partly because the hero may be too young and too naïve to understand what the mentor is saying. It may also be true because the mentor may have so much information to convey that it is difficult to choose which information to give first.

Functions in Story

Mentors can have extremely important roles in your story. They can be instrumental in getting the hero started on his/her quest, such as when Gandalf makes Bilbo pretend to be a thief in order to march off with 13 dwarves on a great adventure in The Hobbit, and then gives Frodo an even more significant assignment in The Lord of the Rings.

Besides imparting wisdom, background information, and setting protagonists on quests, mentors often:

• Train the hero. Consider the time that Luke Skywalker spends with Yoda, learning to be a Jedi Knight.
  
  
• Give comfort and shelter. In an adventure story mentors often provide an important respite from danger and despair. They often also supply a hot meal, a bath, and a chance to sleep.
  
  
• Help in times of great distress. Every now and then, when it seems that all is lost, mentors show up and slay monsters or point out which path the hero should take. Sometimes they do this by actually appearing; another way they can assist is by heroes remembering tools given to them, or words of wisdom that were confusing when heard but which now make sense. However, your heroes should not be rescued by their mentors in every perilous situation; if so, there is nothing left to admire in the hero.
  
  
• Give a sense of order and righteousness in the world of fiction. Whether or not you believe in the divine, the idea of an extremely powerful being occasionally appearing to assist the deserving is extremely comforting.

Key Mentor Plot Problem

A plot issue that needs to be addressed in nearly every story with a mentor is to show why, if the mentor is wise and powerful, s/he is not taking up the quest her/himself. There are nearly as many answers to this question as there are stories. Here are a few:

Dumbledore. Dumbledore actually does do as much of the work himself as he can, destroying one of the Horcruxes, and spending time training Harry Potter. However, the series is set up with Harry Potter as "the Chosen One" -- since Voldemort tried to kill him -- so there is no question but that Harry has to be the one responsible.

Gandalf. Frodo is the first bearer of the Ring to receive it lawfully, unlike the previous owners, who mostly committed murder or other crimes to possess it (even Bilbo took it without the permission of the previous owner). The circumstance of legal, rightful acquisition means that the One Ring has less power over Frodo than it has over previous owners. Furthermore, Gandalf fears the power of the One Ring, so he does not accept responsibility himself, but does what he can to assist Frodo Baggins in the quest.

Mentor/Athena. As Athena is a goddess, she does not fear anything from mortals, but she is concerned about the opinions of other gods. Poseidon, in particular, is annoyed with Odysseus for blinding his son the Cyclops, and this limits, in part, what Athena can do for her favorites.

Other reasons to keep the mentor from helping too much include being too busy; otherwise ineligible; or even lack of interest. You will have to figure out how exactly you are going to get your mentors off your stage.

Death or Removal of the Mentor

Frequently the mentor either dies -- or at least appears to die, as Gandalf does in "The Lord of the Rings." This is emotionally devastating for the hero and often for the readers. I remember reading a kid's review of Harry Potter & the Half-Blood Prince and how angry and upset he was with Rowling (as well as with Severus Snape) for killing off Dumbledore.

Yet removing the mentor can make the story. First, it's emotionally powerful, which is always an opportunity. Second, the removal of the mentor provides new challenges for the hero. S/he has the chance to grow, to struggle on and to learn, to persevere when all seems bleak. In many senses the hero cannot become an adult while the mentor is still around; the removal/death of the mentor is an important part of many coming-of-age stories.

Return of the Mentor

Frequently the mentor, despite being dead or gone, returns in one form or another. Gandalf returns in The Lord of the Rings. Tolkien explains that Gandalf never actually died. Rowling's seventh book, Harry Potter & the Deathly Hallows, is more about Dumbledore than any other novel in that series, despite the character's death in the sixth. We learn about Dumbledore's childhood via memories of his friends, family and Rita Skeeter's biography. We encounter him in Snape's memories, in Harry's near-death experience, and even the portrait in the Headmaster's office at Hogwarts. Furthermore, the three main characters spend chapters speculating about him. I adore the Harry Potter books, but while reading the last volume I could not help thinking that for a dead wizard, Dumbledore sure talked a lot!

There are several reasons that a story may be improved by the return of your mentor. First, the mentor is often the source of explanations. Second, the main character, having gone through the coming-of-age experience, needs to meet the mentor as an adult -- a completion of the circle. Third, the readers often yearn for this, because at the end of the long struggle they want to go home. It is emotionally satisfying to sit down and chat with the mentor, especially in books for children as the Harry Potter series.

Nevertheless, some authors resist resurrecting the dead, and turn instead to other ways to evoke the aura of the mentor. Perhaps there is a letter or a message. Perhaps the mentor appears in a dream or a hallucination. Perhaps the hero takes over the duties of the mentor and lives now in his home. Perhaps the hero simply stares at an old photograph and considers what he would tell him if he could.

Wisdom of the Mentor

The mentor is frequently supposed to be extremely wise. This means that we, as authors, are challenged to be as wise, because we have to give our mentors words to say that that can pass for proverbs. Here are some recent classics:

Yoda: "Do, or do not! There is no try."

Dumbledore: "It is our choices... that show what we truly are, far more than our abilities."

Gandalf: "Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgment."

Aspiring to such pearls may be daunting, but I think we should try! (Unlike Yoda, I respect those who make an effort -- for if you never try, you will never do. Besides, writers can edit.) A few good phrases -- hopefully great ones -- may turn your work into a masterpiece, into a story that resonates with your readers long after they have stopped turning the pages.