Sorry to see you go, Pakky.
I think everyone in this thread guesses Erdrick, but got wrong what was actually going on.
All the evidence points to Luminary as the face from the start. The stage was clearly figured out early on in development. They had a clear-cut plan like with all DLC. They need to figure out the main plan immediately in order to license it properly. It doesn't matter if Sakurai or Square-Enix proposed Erdrick from the start. That wasn't the final decision to begin development with regardless. Also, contrary to popular belief, the Kirby hats are equipment Luminary uses. The headgear does admittedly look similar to the one Erdrick wears, though.
The Spirits don't even focus on DQ3 bar the Erdrick one alone. And that's not even properly called Erdrick either, like the rest of them. The rest are from entirely different games, including 1, 2, 4, 5, and 11. 8 surprisingly only has the Hero spirit, considering the Mouse could've been another good Spirit choice. Though 8 is mainly in as a costume to appeal to the US playerbase. There's a decent amount of love for Dragon Quest 1. The actual Mii costume is specifically Erdrick's Equipment from that game. Which is not really so much a call to Erdrick as being "a costume of the Dragon Quest 1 Hero specifically". It's just the lore obviously is with Erdrick as he's infamous in-universe. Which is understandably why people focus on him as being the most important hero. It's not wrong to think they'd use him as the face, but that wasn't clearly they case.
Hope that explains a bit of the issue with the whole idea Erdrick was intended to be the face. It doesn't add up whatsoever. Only Verge at most leads up to it being true, but he gave no real explanation other than him being in that it doesn't actually provide anything of use other than "well, yeah, a costume exists too" as a proper interpretation of what he knew. Maybe he'll come out with more information if he ever had it. Besides that, to be fair, would someone want to leak anyone other than Erdrick as the face? Probably not, as that would ruin the surprise.