honestly for me the white knight was Ashe in my head. Like in the hunter route we stop her from killing the mayor who turned the princess (now queen in the vampire route) and since we didnt do that in the vampire route and how the white knight is described having freed the princess now queen from her maker and as it is shown in the hunter route that Ashe on multiple occasions freed vampires from the control of the one who turned them on top of said knight being extremely powerfull and able to use the malisphere. To me it made immediatly sense it was her. But that's just me and im just reading too much into it (i really love the game that's why)