In the beginning it seemed like Isabelle was developing, but now it's obvious she's not, or at least it's handled extremely poorly. It's no doubt related to how she's a character added to this manhwa adaptation only, in this role. So, since she's not there in the novel, her development would obviously not be explained. We got a couple of scenes earlier, which were extremely easy for the manhwa author to add, but then nothing. Yet she's still following Frey and the fights are getting more dangerous. Realistically she shouldn't anymore be able to survive. She was three star in the beginning. We might barely be able to assume she's now four star, after using the battling technique, but that should be it. Frey ought to be closing in on peak seven star now, with the apostle exctract. However, as we have seen, Frey doesn't actually help Isabelle to get stronger at all. He has had a few precious potions, but he has gulped down them all by himself, not even sparing a drop for Isabelle. This makes perfect sense if he was alone in the novel.
So, Isabelle is a mere four star (at most) mage. How is she supposed to survive from now on? Or does she magically get stronger with no aid, just to be able to follow Frey? Such game mechanics would be pretty funny, seeing how Frey needed powerful, rare elixirs to quickly level up.