General Demoux was a Captain in Kelsier's army, and later a general in Elend's. Demoux fights a man who believes that Kelsier's rebellion will fail during the first book, and is helped by Kelsier's Allomancy. Despite being almost a member of the crew, Demoux sees Kelsier as a god and is a preacher in the Church of the Survivor.
He is Snapped during the third book by the Mists, and becomes a Seer, being able to burn Atium. However, since Demoux suffered the "mist-sickness" for sixteen days, rather than one, it s more likely he is a full Mistborn (needing one day of recovery for each metal).
It has also been confirmed, by Brandon, that Demoux is also a world hopper, along with Galladon and Hoid, and can be seen in the interludes in The Way of Kings as the man (with the scar) searching for Hoid/Wit.
Sidenote: He was named for Brandon Sanderson's friend, Micah DeMoux, who allowed Brandon to use the name only if his character could "survive and get a girl". Incidentally, Micah is also the person who took Brandon's author photo. See the photo credit on the book.