I'll add that while I am a Ranka fan compard to Sheryl (hey, I like both), I think it's fairly obvious why Ranka plays the victim or in her right mind, plays in a laid back manner in regards to dire situations, whether in romance or in battle. It's probably the victim complex due to her past and maybe even *gasp* her actual personality to do with that. I don't think it's inherently faulty of her to do so, just as much Sheryl's bossiness and cockiness can get her into trouble too. They obviously both have their pros and cons but I can relate to Ranka a bit more because my attitude is just like her in real life AND she's so insanely cute.
All that said, I don't think Sheryl's obstinate attitude could save her from the likes of the Vajra like that. It's fairly obvious that the show is putting Ranka's helpless attitude right on top of her helpless situations.