I'm not saying that she is weak (have no idea about that), I am saying that the way she is shown lack depth and her anime character lack streight as a character not as a her own character...well then again she simply have no character beside the cuteness intended for fan service....
Sure she is traumatized, or at least she should be, but the way anime (haven't read manga and do not intend to) shown her up to ep 2 is sooooooo weak that nothing of her story shows up in her character its just a story.
Yet, all of that is quite intentional, as it helps build what's yet to come.
The whole point of these first three episodes are to introduce us to the characters and explaining how Rea ends up becoming a zombie (which I don't think anyone
didn't expect). All subsequent episodes will show us Rea's transformation and what being a zombie is really like. And as said, Ranko is obviously the secondary love interest, whom Chihiro doesn't give two shits about, but is there to 'spice things up'. If you ask me, if anyone is going to a forgettable character, it will be
her, not Rea.
I will be precise and blame manga then, thou it usually comes out that anime ruins the manga by not realizing the depth of manga, just as movies ruin books. Haven't read any manga in a while and do not intend to read this one. If I read manga its horror manga, a proper horror manga not cuteness based crap.
If the words of the mangaka are anything to go by, he'll try anything he can to make sure the anime is going the way he wants it to (i.e. following the manga as closely as possible, with only some minor stuff added to make the story more coherent). So, if you're worried that this show will miss the subtleties and depth the original source has, I think it'll be fine.
BTW, this is NOT, I repeat,
NOT a horror story. It's a romantic comedy, so if that alone puts you off, you might want to get of here and find something else to watch.