Thanks so much for the detailed comment, and I'm so glad you loved the game! :DDD
I'm not going to answer too many questions, as I feel like it's a lot more fun to let people discuss their own theories rather than spell everything out, and I do have loose ideas for a possible sequel and/or maybe a side-game, but those are very much in a "if they happen, they happen, but it probably won't be really soon" state, as I have other games I want to make, too!
Again, I'm going to pass on answering most of the questions, but I'll answer a few:
"I didn't encounter the invisible guy in any area yet. Is he an important piece to the story?"
He is completely an easter egg. He has no importance to the story and is more of a "wonder if anyone will find him" secret.
"Is the Anima Eater(The one that consumed amethyst) some kind of natural parasitic evocation or an artificial one made by the inner evocations? If sequel comes, will Cephiti get a redemption arc and possibly be a key connector to some mysteries still unsolved? "
I have ideas about possibly working Cephiti and maybe even Anima Eater into future games, so maybe there'll be a chance to learn more in the future :D
And yes, unfortunately you have to leave "the princess" to her fate.
Thanks again for getting so into this game and the detailed post! :DDDD