Sounds like you are talking about the Azura's Star quest where you can get Aranea Ienith at the end. She's one of the ones I'm saving for when I'm higher level. I suppose I could use the console to fix her level as I level up since I'm playing the PC version, but that sounds annoying. Oh well, if I'm going to the mage college then I may end up causing her to load anyway. I'm not sure how far you have to stay away from an NPC before the game loads them, but at that point her level becomes fixed at whatever level I am at and the only way that I know of to fix the bug is with the console. She is definitely a badass, though. On my thief character she is my follower and I've seen her regularly take on dragons by herself.