Oh golly gee. I wonder who that saintess is?
That aside...okay, so that narrows things down a little. Graham and that guy are one and the same, whether he's a reincarnation with his own memories, possessed/replaced by the demon lord, or just from a missing link in Louisa's memories.
Of those, I think the second is the least likely, since Graham seems to act and care about the same things as before, though it might be less possession and more merging.
...Huh. A new thought strikes me. What if the goddess or some other person is also reincarnating down the line, and she's the one that's always seducing/scheming the Hero away from his lover? Either for evil reasons, or simply because that soul loves the Hero too.