This game was excellent! Instead of being called a demo, I would have called it Magical Warrior Diamond Hearts season 1, because it really felt like you were watching (or playing through) a season of a magical girls anime. It was lengthy in a good way, and felt complete. You got to play through a story arc from start to finish, which is rare in VN demos but is something I appreciate. The art felt authentic to the magical girl anime style, especially the transformation art.
Some personal opinions:
My favorite character by far was Valerie, she was such a good example of a successful main character. She had her own distinct personality, and was still likeable, but had her flaws too. I also loved Clover and Liam to death! The characters I liked in order then were Alex, Sophie, then Ethan.
I was pretty ambivalent towards Zacharie. I wanted to like him but he just didn't have enough of a personality to me. I also would have altered his character design, I would have preferred he looked more androgynous, to me he looked 100% female. Some height and broad shoulders would have been a good addition.
I actively disliked Diana and Amber (sorry!). They didn't mesh well with the group and didn't seem to need to be there. Having Amber take up a place on the magical girl team annoyed me because I would have rather that spot be taken up by a new classmate of Valerie's. Amber was also confusing in the game, because she simultaneously is addressed by both Amber and Amy, and her hair when transformed (when you see her first) is a natural blonde, but her hair untransformed is dark brown- it took me a while to realize that she was the same character.
The writing was excellent, the plot was well-crafted and was very accurate to the magical girls genre.
Overall one of the best demos on itch.io to date and I highly recommend this game.