Some of the greatest mentors are those that have passed to the great beyond, like Mufasa or Obi-Wan Kenobi. Peter Parker (aka Spider-Man) had his Uncle Ben. When Peter fails to pursue a thug his uncle is killed, leaving Spidey with guilt that only becoming a masked superhero can resolve. Never has a character (Uncle Ben) appeared so little and yet left such an impact.  

Top quote:

Ben Parker: “With great power comes great responsibility”