Who is the mentor in Twilight?

If by mentor, you mean the person who had the most influence on telling this story on film, then the mentor would be a group of executive producers, including -- from IMDB --

• Marty Bowen, executive producer;

  Wyck Godfrey, producer;

  Michele Imperato, executive producer (as Michele Imperato Stabile);

  Jamie Marshall, co-producer;

  Greg Mooradian, producer;

  Mark Morgan, producer;

  Guy Oseary, executive producer;

  Karen Rosenfelt, executive producer;

  Patrick Thomas Smith, associate producer;

and people who work with:

• Summit Entertainment (presents)
• Temple Hill Entertainment (as Temple Hill)
• Maverick Films (as Maverick) (in association with)
• Imprint Entertainment (as Imprint) (in association with)
• Goldcrest Pictures (in association with)
• Twilight Productions

Every 'producer' had influence on this film, based on either championing the story as cinematic, raising money for the production, or working on set as a producer.