I got around to playing the demo, and I'm liking what I'm seeing so far after playing with all three characters. The battle sprites and the character talk portraits are nicely drawn. I also love Gremory's and Vepar's smug faces.
For feedback:
The music was a bit load even in the lowest setting. I did see you know about it, i just wanted to mention it anyway to have it in my comment for reference.
I liked how you drew custom sprites for battle even with using RPG maker enemies. Though I would suggest also doing custom world sprites and maybe custom stuff for the world itself. Cuz the default sprites don't quite match. Along with the imp and bat looking really big next to our protagonists. That, and anyone who's played RPG maker games for a while can spot the default stuff a mile away. I understand using the default stuff. But I also think the more custom stuff you use, the better. Both to show the game is yours and to make it stand out among the countless other RPG maker games.
I had one bug; Gremory's save icon didn't show up when the other two did.
You said Gremory was the hardest, but I found I had to restore HP and MP the most with Vepar. I understand this demo is basically just the intro, so that could change in the future. But just going off first impressions that's what I got.
All n' all, I'm defiantly looking forward to the game. And I wish you the best in regards to development.
(Also shout out to Vepar's thighs in the battle sprite, those things look so plump I swear I could reach out and grab them)