The motives of the monster seems twisted, both acting on Furuka’s will with the strong gust, as well as hoarding Kon away, who just wishes Furuka to be safe. Problem is what is her family thinking, the world seems to have forgotten Kon, but she certainly seemed to have a life before Furuka showed up, right? Could be like Granblem tho. . .Wasn't there a girl who looked like Kon from a photo from way back too? This?
Maybe who we know as Kon is that original sacrificed girl or a recreation and used like a lure by the monster over the years? Or perhaps not a lure or anything malicious but it's only way to interact with humans using the one it was given. Plus it's protective of them with those super power "wind gusts". That grandmother with the journal fell in love then escaped the cave and so did the protagonist.