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I just finished the true ending of Cold Steel 4, and I saw a walkthrough video of a streamer who skipped the (???) quest and only got the normal ending. After they load the save data before the battle with Osborne, they warp to the outer side of the dungeon (Tuatha de dannan or something?) and found a little purple(or is it red?) portal.
When Rean touches it, there's a text box told the player they can change the fate to a certain extent, and then there's a voice said "Now go" (I guess if I'm not mistaken) then Rean back to the past and got call from Toval(the beginning of the secret quest).
My first guess about that voice, is that the Aidios' voice? Or maybe (Idk why I thought of this), Ouroboros' Grandmaster...? (Whose voice is that? (If you know the voice actor EN and JP that'd be great too).