I'm rereading the series and passed the part in book one where Elend was going to be assassinated. Vin jumps in and battles a handful of mistings and two Mistborn. One of the Mistborn, of course, was Shan. I wasn't able to find who the other one was. All the info on him I could find was that he was male.
However, having just finished book two.... Its Zane isn't it? Male, Mistborn, working for Straff. Seems right. The big point against it being Zane is how easily Vin beat him, but she could have just been lucky.
Also would she have recognized him in book two if this was the case? It was dark when they fought and I don't remember if they were wearing masks?
Has everyone already figured this out? Is this wrong? I can't stop thinking about it lol.