The possessed girl can only be seen in the dark.
...
So how come near the beginning, when that little boy comes crashing through the ceiling, we still see him even in the light? (As I presume all the kids in this room were the original possessed)
He is possesed yes. But he's just a simple zombie, not the real medeira demon
shareNo, he is infected--not possessed. The demon is trapped in a prison of darkness. But it can infect others who are free to roam about. The infected children are perfectly visible in light, as are infected adults.
Only Medeiros or Angela are possessed.