A mentor is the character who trains, guides, or equips the protagonist: a teacher, master, coach, or elder whose knowledge the hero inherits. Mentors explain the world's rules to the audience by teaching them to the hero.
The mentor's dramatic function is transmission. Through them the world's rules become teachable: the power system's grammar, the profession's ethics, the history the hero was born too late to see. A good mentor scene does the work of a chapter of exposition while feeling like a relationship, which is why the type survives every genre cycle.
Mentors are also structural sacrifices. The mentor's ceiling exists so the hero can be seen passing it, and stories routinely retire, defeat, or remove the mentor to force the student into the open. The pattern is so established that a long-lived, thriving mentor reads as a deliberate subversion.
The archetype has deep roots in the martial-arts and master-apprentice traditions that fed early manga. On this site, who trained whom is treated as core series knowledge, a load-bearing fact of a canon sheet, because so much of a cast's ability and philosophy is explained by the line of teaching they stand in. The mentor's own master, when a story bothers to show one, extends the line backward and quietly tells the audience that the hero's training is one link in a much older chain.
In use"The retired master who takes the hero in, sets the training, and withholds the final lesson is the mentor at work."