Usually right at the point that the heroine of your story is deciding whether or not to accept the call, the Mentor comes into the picture. The mentor can be an old man or an old woman, although this has become rather cliche. I would suggest you make your mentor someone least likely to be a mentor. This will add a nice twist to your plot.
The Mentor’s role is to prepare your heroine for her Call to Adventure. The Mentor usually gives advice or even an item for the heroine to use on her journey. This item is symbolic and has a metaphorical meaning in your story.
While your heroine is on her journey, the Mentor’s advice will inspire her at times when things are going badly. You can even reintroduce your Mentor at a critical point in the story.
Remember that although you may use a Mentor in your story, the heroine must solve her own problems. The Mentor serves simply as a counselor, but the final decisions are left to the heroine.