Could be a child. Children are not an uncommon sight in marriage ceremonies. They're sometimes playful, sometimes shy. Both could lead one to hide behind his parents. The size of a child could fit behind the giant man on the left without too much contorsions.
Anyway, if any contorsion is required, I know of a posture in Chinese martial arts which would allow an adult sized person to hide behind someone else without bending. It is called the "Yuhuan Bu", which means "the jade circle stance" (
ah ! Chinese poetry !) : you advance with a left or right step, your front foot bent at about 45 to 90° towards the side (that is : your front foot doesn't point straight forward, it points towards the side). Your front leg squats with a 90° angle, which lowers your whole body down. The other foot remains in place (but usually, only the toes touch the ground. So basically, you keep facing straight ahead, while it seems your front foot looks to the right or left side. It is a very stable position, allowing your torso to bend ont either side, without requiring to make your butt protrude in any direction.
I do not imply that the guy hidden here is a martial artist, only that it is technically possible to hide behind the black clad man without letting one's buttocks protrude. For me, what we have here is a child, and certainly not a ghost.
By the way, if you look at the picture, you can see that the man on the left doesn't look towards the photographer. He looks downwards toward the right of the picture. So he was either looking at the eskimo-style lady (
thinking at how pretty she was, perhaps), or at the little boy playing "hide and seek". In any case, he doesn't seem very frightened. If he had seen a lurking demon, I doubt he would display such a ravished look.