Free transformation game at https://www.pixiv.net/member_illust.php?mode=medium&illust_id=64892780 .

Do you know if you get anything from beating the demon girl? It seems doable but I also don't feel like trying because I feel like theres some text queues I'm missing with it being in Japanese.

Base +6

Not much. If you beat the oni, you get a thank you card with a description that basically says "thank you for playing" for doing it. It's mostly for the satisfaction of having done it, I think.

Base +6

Lihama Daka released an update to the game. It adds a new area, a few new enemies (though it removes the Green Haired Girl), and appears to remove some of the skills. Google Translate implies that it's intended to be a demo for a larget game. https://www.pixiv.net/member_illust.php?mode=medium&illust_id=65687323

Base +6

Full game released: http://www.dlsite.com/maniax/work/=/product_id/RJ217347.html

Base +22