WMG / Beyond: Two Souls

There won't be any multiple endings.
Well, since this game would have only one playable character, 'nuff said.
  • How could a Quantic Dream game only ever have one ending?
  • And what does the number of playable characters have to do with whether it has multiple endings?
  • This is Jossed, it does have multiple endings.

Aiden is the ghost of a member of Jodi's family
It's the most obvious guess but someone had to say it.
  • Confirmed. Aiden is the ghost of Jodi's stillborn twin brother.

Jodi was involved in some sort of secret government research
...as a test subject, and now they want her back. This one goes to Quantic Dreams' previous incursion into the conspiracy genre.
  • Confirmed.

Jodie will not be the only person that can see Aiden.
Perhaps at some point in the game there may be a particular person who can see Aiden for various reasons.
  • Confirmed. At one point, Jodie enters a mental hospital; if you switch to Aiden and enter the patients' rooms, some will comment that they can see him.

Aiden is Jodi from a Bad Future, come to Set Right What Once Went Wrong by protecting her past self.
  • Jossed.

Aiden isn't a ghost or person from the future, but is actually an evil entity
The game is confirmed to deal with the afterlife. But nobody ever said that Aiden HAS to be a spirit of some sort. He could very easily be some other entity trying to gain Jodi's trust for an ulterior motive.
  • Jossed. Aiden is the ghost of Jodi's stillborn twin brother.

Aiden's human form is

Norah Gray kept her maiden name
  • The last name of Jodie's dad was Joestar. She was cursed to have bizarre adventures from the start. Her star birthmark was removed at the request of her father, who hoped that the family curse would be averted by the gesture.
    • Aiden isn't actually Jodie's brother as an entity, he's her Stand that she controls semi-consciously. Every Stand has the basic function of self-preservation, which is why Aiden does things like strangling that kid during the snowball fight or keeping Jodie from shooting herself. He also doesn't have his own will, he's actually acting out Jodie's subconscious desires.
    • Jodie grew up in an abusive household and created a split personality, Aiden, to deal with the abuse. When she manifested as a Stand user she began to see her stand as that imaginary person trying to protect her. This let her lash out at her tormentors without having to take personal responsibility for her actions.

WMG:Aiden doesn't actually exist.
  • Rather he is manifestation of Jodie's subconscious and lateen psychic powers.

The other ghosts are real but only speak to Jodi because she's a medium and can't interact on the material plane without her