You will note that none of the next 7 Elements are "the main character." This is because every main character is more than just the position they occupy in the story, and they're almost never alone. So while any of the next few character types might be your protagonist, they're more likely to make up your supporting cast.
Today's Element is the Mentor. You've probably seen this character in a number of films, TV shows, video games, and novels.
Each of the four figures above have something in common - they are all Mentors, teaching or guiding the main character through a set of new skills relevant to the character's life at that point or the character's future. Whether the subject being taught is physical prowess, mental acuity, history, or morality, the Mentor's guidance always plays a relatively major role in the main character's personal development.
In some plots, the Mentor may not be around for very long. Sometimes the Mentor is only present for only a few scenes before the circumstances of the plot call for a dramatic sacrifice or a duel to the death.
That said, the Mentor can be an extremely useful character to have in your story, as they can add a source of wisdom and information to the main character's journey. This is especially relevant if the main character starts off in a state of confusion or isn't familiar with the situation/world/position he's been tossed into.
What mentor characters are in your favorite media?