I was headed to Darklight Tower, just below and to the left of Riften, to clean out all the witches and Hagravens there. I pretty much ransacked the place in a previous save but had so much fun doing it I reverted to an old save and began playing on that to once again attack Darklight Tower at later time. Well I'm very fortunate that I did go back to an old save because as I was sneaking up behind the first witch with my dagger out preparing to slit her throat, thanks to the awesome kill-cam sequence, she began talking to me. Apparently she was rebelling against the witches there because they told her to find a human sacrifice and she just couldn't do it.

The quest there in Darklight Tower is called Repentence and it's started by speaking to the first witch you find named Illia. It's a fairly quick and easy quest but I was surprised because I've never heard anything about it before, plus in a previous save, I killed everyone there.

Anyway, after completion of the quest you gain Illia as a follower, which is really awesome. What I'm wondering is if I don't take her with me now but need her later, can she always be found at Darklight Tower?

I use her ALL the time. I always take her after Repentance because she does disappear. I'm not sure about her disappearing after the mission, but if you dismiss her or if she gets forcibly removed, you can never get her again.

Ah, that svcks. I've never really used too many followers in Skyrim because I always go for a sneak attack and they usually ruin that. Since I'm not to familiar with the ways of a follower, when you have one with you, they follow you everywhere and when you tell them to wait somewhere they will wait until you tell them otherwise, right? Kind of like in Fallout 3? I kept Fawkes in my house for a long long time and would take him out for adventures every so often, lol.

I'll second Illia as a follower. My current character got her as a follower last night. She is good. She focuses on ice magic. So far my only complaints are her use of frost shield and she tends to lag behind.