Beyond: Two Souls

Aiden is an Entity that has been linked to Jodie Holmes since birth. They are connected by an Ethereal Tether, that can be seen when switching to Aiden's view. This tether acts as a spiritual link between them that keeps Aiden bound to Jodie. Though he can travel farther away in some situations, the separation causes Jodie excessive amounts of pain and lasting disorientation.
Jodie and Aiden seem to age in sync, for as Jodie grows older, Aiden grows stronger, and their bond grows greater. As the story progresses, it is revealed that Aiden is Jodie's stillborn twin brother. Jodie was delivered safely from her mother's womb, but her brother was strangled by the umbilical cord. Rather than passing on, his soul became linked to Jodie and remained that way, growing up as if it occupied a physical body.