A cute, short game about a gay dork and Tsundere Demon Prince. What more about it?
It might seemed rushed because the game was initially planned for a game jam, so a production during a limited time and shortage of assets; the reason why it's also on the shorter side. As someone who experienced rushes before for projects, I don't judge the creator for that. It's all about being done with the project. Polishes may or may not be allowed afterwards.
I also wouldn't expect from one writer who also has to do the GUI and coding and general marketing as well direction of the team to write a long, fleshed out story in a month. Credits are a nice thing to see who worked on what and I see clearly that the developer took on quite a bit for Demon Kiss.
Basically this a short, sweet story between the two main characters without unnecessary drama that shouldn't be read into that deep. Sure, this doesn't excuse the lacking points like some rushed writing, but who came for a full lore and drama filled story anyway? You just want some quick fluff sometimes to fill in that dark hole in your chest to warm you up.
And a big plus to it being dorky cute and not Yaoi-sexual. I demand a sequel! Maybe even for Vanessa, since some people seemed to be a fan of hers in the comments. Though it won't be BL anymore, huh.