She has 47% magic resist and can go even higher thank to spellblade class.
I rarely see Whisper on arena, and when I do she never do anything to make me consider her as a threat. I mean she certainly has some crazy resistances and stuff, but I think if units like Aileen is on the lowest tier, then so should Whisper.
The thing with Sasayaki is that she requires high-end gear to really shine as a tank. But when you have the needed Esper, rings, VCs, etc., you can pick whatever she needs to defend against and she will NOT get hurt by it.
Aileen is B tier because you can give her the best gear in the game and she will still not be as good as the others.