[Von Himmel]

It could be the player...

Spoiler:

But I also read a theory in gamefaqs that it's supposed to be a new character who has the same power as Sigma and Junpei since '?' mentioned 'his body' in the dialogue. That implies he has a real body somewhere or in different time and he/she got himself/herself in Kyle's body.

I personally think that he's a blick winkel entity and only said 'his body' since this entity hasn't got a clue about his 4th dimensional existence yet. Basically, I mean that this entity still think that he's stuck in a human body (and eventually, thinks himself as human) eventhough he's supposed to be something that lives in 4th dimension.