Interesting use of the water effect, but I hope it doesn't become a trend for anime in general.

Ufotable's gotten better at expressive faces since the last time I checked out their stuff. The CGI is still jarring, but it's not the worst out there.

I wonder how they'll get around the sister problem. She obviously hibernating because she hasn't eaten any humans. We're not told of any substitute food. Slayed demons turn to ashes, so she'll have to be given a fresh one somehow if we're to go down that route.

Eating humans would be a no-go, since this Morphed Demon would suggest to us that the more humans you eat, the less human (and more demonic) you become. The MC may at one stage give her some of his own blood, but that's as far as it'd go.