Feel free to shoot this down, just another random thought, what if the mist spirit is the lord ruler? I know the y existed at the same time back in alendi's day, but maybe that mist spirit vanished once there was no danger of ruin being released, and now that there is danger again the lord ruler replaced that old spirit. Doesn't really make sense, but waht do you guys think of it??
It seems to me that the most likely guess for the Mist Spirit is Preservation. First, we can assume that Preservation and Ruin are opposites. It seems like Ruin is an actual entity of some sort, since he talks and escapes. Not quite a person, but also not purely a force. Something in between. if that's the case, it's likely that Preservation is something similar.
We also know that the spirit seems to want to keep people away from the Well, which is where Ruin escapes from. If they are opposites, it would make sense that Preservation would be trying to prevent people from going there.
I think all that is fairly well supported, but there is also some more "crazy" links that can be made.
First, the Mist Spirit seems to know what will make Elend a Mistborn. There is an obvious connection between
Mist Spirit and
Mistborn and the
Mist.
If you take the simple route, and just tie all that togeather. It would mean that Preservation is the Mist Spirit, the Mist Spirit is also the Mist, and Mistborn are closely related to all that. So that also means Preservation is the mists (or has something to do with it) and Mistborn are part of Preservation.