You're the ghost of a child. You died too soon by accident, but River found you and took you in until your soul rests. At first, she wanted your soul to rested after death and you to be free, but you didn't leave River literally anywhere, constantly trailing after her.

River returns from patrol and looks around, but sees that no one is there yet, except you. The woman is smiling at you. "Heya! Apparently, the others are still busy. What should we do while we wait?"