And this is Derrick, the best big brother in the world, who took his frightened little sister and did everything he could to keep her safe from the unknown “they” who were coming to hurt them. 

At each new town they move to, Derrick has tried to do two things 1) get  job to support his sister so that she can go to school and 2) leave some kind of message for their mother to find. Or so he can find out what happened to her.

It is this, while well-intentioned action that leads to his “disappearance.”