After beating the game, without finishing any of the post game content, I lent it to my boyfriend. But I forgot that I never learned who Horace really was! I don't care about discovering it on my own, so someone please just tell me lol. I had a feeling that he was another one of Shadars (Luciens) past lives, since he sort of resembles Oliver and he also is obviously from Nazcaa. So who is he?
Horace is the "Sage of the Ages", just as he has been claimed the whole game. Not only was he the second greatest wizard who lived during the Wizard's King time, he also served as Cassiopea's personal court wizard AND as Gallus, the 10th Zodiarch. In addition to all of the above, he wrote the Wizard's Companion himself, as an instructional piece to give to Princess Pea (Cassiopea).
Unfortunately, the Zodiarchy (minus Horace/Gallus) conspired to kill the Wizard King, and in doing so, usurped the power of the throne from the monarchy. When this happened, they also killed anyone who stood in their way (which also included Horace). The real kicker, is that Horace was not able to explain to Cassiopea that the "Manna", or "Ashes of Resurrection" spell, had a grave consequence if it was ever used. Needless to say, Cassiopea ends up using it and killing everyone over time, while staying alive. After all of this, eventually the ghost of the Wizard King takes the guise as "Gallus", serving and watching for a time to help Cassiopea.
That is pretty much it, though I may have missed some smaller details.
Not to nitpick, but Horace needed to explain to her HOW to use the Ashes of Resurrection spell, not to tell her that it cannot be used at all. This is further proved by her having complete mastery of the spell during the final battle. I'm sure this causes several huge plot issues, but I guess what's done is done.