Not quite sure, I mean we did release parts of his soul from imprisonment in Mage Tower. I would assume that the Hauberk that we meet in front of Demon Prison would be the same Hauberk from Mage Tower, since the person that he possessed at that time was a mage and the personality of that particular Hauberk doesn’t quite match one you meet at Sutatis. Unless both of those spirits got reunited, of course.
But, on that point, he would definitely recognize you as the one that helped him. Then again, he might’ve just pretended to not know you, as to fool you into helping him once more.
As for why we’d help a demon lord, well, the first time that we’d encounter him(basing from where the area’s located and technically it is the earliest time that we would meet him), he’s trying to save the life of the pilgrim that he possessed. So, us being the good natured Revelator, decides to assist him. Plus, hearing about him not wanting to be evil and just wanting to be left alone tugs at our Revelator heartstrings(I assume).