So everyone seems to assume that Rey is either the child of Han and Lea or Luke. I guess it's because of the books that followed episode 6 and mostly focused them, but this is a new story that won't be following the books. So I want to propose a new speculation. Rey is Obi-wan Kenobi's grandaughter. Anyone who has watched Clone Wars will know Obi-wan had a relationship with Duchess Satine Kryze leader of the Mandalorians. In fact when Obi-Wan was still a padawan he and Qui-Gon protected her for over a year. Obi-Wan and Satine grew so close that Obi-wan stated he would have renounced his vows to the jedi order had she asked. Now Qui-Gon was certainly no stickler for the rules, but fathering a child is in fact not against the Jedi code. The attachment that usually comes with such things is the issue. So Obi-wan could easily have fathered a child with Satine and remained a Jedi so long as he was detached enough from the situation. This child being of both mandalorian and Jedi descent would have to be kept a secret. It would have been an affront to the mandalorians seeking to return to the old ways and a target of assassination as a result. Bonus speculation: What if Rey has been trained as a Jedi Knight by Ghost Obi-wan?