Why does Ghast exist? Why is he invisible if it only creates difficulty that forces others to play Porto?
It's implied in the story that demons rose up because of humans ripping the earth apart. Ghast is an invisible ghost/demon who can breath fire. Is Ghast a rebel demon who rose up against his brothers and sisters, or is he a dead miner? Maybe the unknown miner is who Ghast was originally, before he died. In this theory I'm assuming that Ghast is a ghost.
In an interview, Karvina said:
"Killswitch was designed to be a unique playing experience....The fates of Porto and her beloved Ghast are as unknowable as our own..."
Porto says in the game "I knew this man when he was alive." Maybe she's hinting at a relationship between the two. Given, it may not be a romantic one, but it was certainly close.
When Porto leaves the mine, it might go white because she's seeing the light and the wrong of the doings of humans. Who knows what could happen when Ghast leaves?
Maybe if anyone completes the game as Ghast, Karvina makes contact with them for whatever purpose they serve and whatever the "eyes" are for the "acolytes."
That's it for now.