Wow, thanks man I actually never tried r18's image search feature though I've been using it for many years. And I'm now 99% sure it's Harukawa Sesera.
Here's a better image from trailer.
View attachment 3057765
But I don't get why they don't mention her in credits. She has done quite a few titles and even in this particular movie, she isn't side character, but lead with Wakaba Onoue.