I read chapters 1-8 and a little bit of 19 and 20! I usually do this to compare and contrast where you started vs your current progress but I see that you've only begun publishing since the beginning of the month. Progress is still progress but it's something I keep in mind in regards to giving critique (ie I don't want to harp on habits you no longer do because I only read the beginning and not your most recent chapters). anyways, onto my thoughts!
I don't usually read isekai novels so I'm not super familiar with the genre, but I think this is an interesting take where Vivia isn't from our world or a separate world but rather she's reincarnating into someone who already exists in her current world. It leaves a lot for mystery and discovery without having the protagonist feel like a total fish out of water, I personally think its a nice balance.
I like the way you write dialogue, the banter feels nice and when we're in the meat of a scene I feel like it's the strongest aspect of your novel so far. You obviously have a specific vision and dynamic for your characters and it comes across well in their patterns of speech.
I think a general critique is just pacing! I feel like you want to jump into the best moments super quickly which leads readers feeling like they were rushed along other parts of the story just to have a cute moment between two characters we don't know much of? And it's unfortunate because when you decide to write a very descriptive scene, it's very beautiful! But at this moment it feels very all or nothing. You have the chops to write good dialogue and writing vivid imagery, I just feel like you need to marry the two and slow down the pace to really let readers enjoy the world you're building.
I felt this especially in the beginning because we didn't really get to know Vivia that well before she died, especially because there are facts about her that feel like they are abruptly brought up (ie the fact that she had a master who gave her wisdom about or that she knows the demon language). There could have been more natural ways for these facts to be brought up that fleshed out her character rather than it feeling kind of like an asspull for convenient plot sake. Information dumping isn't information dumping if you strategically and thoughtfully place information sprinkled throughout the story!
One final note regarding writing syntax, I'd be careful not to repeat bits of information in differently worded ways. For example, in chapter 7 you state from a narrative point of view that Vivia's surprised about how beautiful the demon realm is and then after you describe it, you have her think it in her head? Unless she said it outloud and it sparked a conversation related to it between her and Zerath, I don't think it was super necessary. And this occurred a few times in other chapters as well, so just something to watch out for.
Anyways, this became way longer than I thought, but I hope this was helpful in some way shape or form! Overall, I think you've got a great core idea and potential to be a great writer, you just need some more practice and then you'll be able to really build grand worlds from just your words 
Not a novel, but here's my comic! I went back and re-did the first 3 episodes (and added a prologue lol) so starting on episode 6 would be super old art, but I'd appreciate it if you skipped to a later episode and shared your thoughts on some of the more recent parts? No pressure though, any sort of thoughts you may want to share would be appreciated!
