User blog comment:Caudyr/The Book of Psalms and Its Various References/@comment-24461718-20150522032753/@comment-4784862-20150522041028

Yeah, it seems that he's considered "Mythology: Main Cast" or something like that for the "Legend of the Black Eclipse Demon"...but I can't be certain on that until I check the raws for earlier days...which I'm not doing, yet. :D

I don't think that the bosses are necessarily a sidequest, but more like notable "characters" that the character is rewarded for defeating in certain ways. Kind of like how Rou's fight with Avenger was recorded and everything.