Certainly, my dismissal of Isabelle was sloppy and poorly reasoned. So let's give a more careful reasoning.
Let's suppose that you had Charlotte, Isabelle, Angela, and Seleia equally powered up. Is there any circumstance at all in which you'd use Isabelle over the others and that it matters which one you use? Maybe there is if you know your opponent's exact accuracy and block ratings so that you know exactly which girls will have your own accuracy and block ratings over the relevant thresholds, but that's atypical. The fire gunsmen Yjer and Elnora that Isabelle counters are handily destroyed by the others, too--and, for that matter, readily beaten by Naladia, so you don't need anything fancy to counter them.
In most circumstances, Isabelle is much weaker than Charlotte. Occasionally the difference between Charlotte's physical damage and Isabelle's magic damage is important, but that's what Angela is for. Seleia is easier to power up than the others, especially because you can readily get her weapon to seven stars by raiding. The natural three star water girls are equivalent in difficulty of acquisition. So why would you spend your precious resources on leveling up Isabelle rather than one of the other, stronger girls?
I'll concede that I don't use Isabelle myself. But Isabelle is one of the girls that I've long been happy to see an opponent use in arena, as she's easy to beat. That's not just now that I have Eremia. Charlotte destroys her, too. Before I had Charlotte or Eremia, Chharodi could handle her. Even six months ago, Isabelle wasn't a strong option.
Coming back to Eremia, the ease with which Eremia handles Isabelle is unusual for girls who get used in arena. No girl takes out Perille nearly that easily. Or Charlotte. Or Leina, Seleia, Chharodi, Eniel, Yidhra, Feita, Angela, Ceilim, Khalis, Claire, Summer Naladia... well, you get the point. Eremia dispatches a same power rating boosted to eight stars Isabelle so quickly that it's almost like fighting 3 on 2. That simply doesn't happen to relatively strong girls--and it's not like Eremia is unusually strong herself.
Eremia destroying Isabelle might be the single fastest, most reliable arena duel win that I commonly see. Leina killing Trisha is the nearest comparable, and people seem to realize that Trisha is terrible. Isabelle isn't nearly as bad as Trisha, but she neither is she good enough to justify spending scarce universal crystals on.