I'm currently putting my money on a "mother of monsters" deal. Giantesses, as far as I'm aware are biologically immortal, so she could be some absolutely primordial being.
It's possible she's actually related to Ophelia, come to think of it- when hiding in her room, the MC smelled "a familiar scent". How could that be possible except if he smelled it before his memories began, back when he was a prisoner still? Couple that with the fact it was dark and she was massive- just like Ophelia during her introduction- and there's a pretty good possibility there's some relation I think.
It's possible she's actually related to Ophelia, come to think of it- when hiding in her room, the MC smelled "a familiar scent". How could that be possible except if he smelled it before his memories began, back when he was a prisoner still? Couple that with the fact it was dark and she was massive- just like Ophelia during her introduction- and there's a pretty good possibility there's some relation I think.