The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at the Trope Launch Pad.
eroock
Since: Sep, 2012
BeerBaron
Since: Mar, 2012

A minor or supporting character who nevertheless plays an important role by introducing the protagonists to the plot. He may be a quest giver or oracle, or someone who introduces the protagonist to his True Companions. A very general trope in mythic and fantasy stories along the lines of Threshold Guardians (but different). Related to The Chooser of the One but somewhat broader.
For example, in The Lord of the Rings, Bilbo Baggins kicks off Frodo's quest but doesn't join. Gandalf plays a similar role in The Hobbit. Hagrid in Harry Potter fulfills a similar role in introducing Harry to the main Hogwarts-based plot.