Why was Johannes David's mentor? Is he a relative?

As far as we know, David and Johannes are not related. We are never apprised of Johannes' last name, nor, his nationality or religion over the course of the text. None-the-less, Johannes is a good man, who looks out for and mentors a young boy, who is all alone in the world. Johannes is well educated and intelligent. In mentoring David, he is teaching him in the same way a father teaches his child.