Marsh is Kelsier's brother. He was originally a Seeker. He was part of the skaa rebellion, but eventually handed control of it over to Yeden as he felt it was a futile hope to keep fighting. He agreed to help Kelsier in his plan to overthrow The Final Empire and the Lord Ruler under the impression he would die. He then spied in the Steel Ministry for a time, before becoming a steel inquisitor. He then helped defeat the Lord Ruler. In the next book, he was slowly corrupted by Ruin. In the third book, he is completely controlled by Ruin except for a crucial piece of him that is self controlled. He then breaks Ruin's control at a crucial time, allowing him to free Vin, therefore allowing her to become Preservation. It is not stated what happens to him at the end of the series, but later being revealed as remain alive and active 300 years later in The Alloy of Law due to his ability of Fermuchemy and Allomancy gained by Hemalurgy. He is now called Ironeyes, and has become a legendary figure.
Marsh has a very extensive history, both in what happens to him before and after the series is set.
Marsh was selected by the Steel Inquisitors to become a new Inquisitor. Kelsier and the others thought that Marsh had been killed by the Inquisitors and this lead in part to Kelsier's decision to confront the Lord Ruler. After Kelsier died, when Vin fought the Lord Ruler, Marsh killed the other Steel Inquisitors in Luthadel and saved Vin's life.
After the fall of the Lord Ruler, Marsh eventually came under control of Ruin, as Ruin used the Hemalurgic nature of the Inquisitors to control them. While controlled, Marsh fought with Sazed to prevent Sazed from alerting Vin to the true nature of the prophecies.
Eventually, Marsh proved critical to stopping Ruin when he briefly resisted Ruin's control long enough to remove Vin's hemalurgically charged earring, thus allowing her to directly access Preservation's power.
Marsh is, by all accounts, a serious and no nonsense man. He is Kelsier's older brother, and frowned upon Kelsier's criminal activities.
Another thing about Marsh is that he once greatly loved a woman. Unfortunately for him, this woman was Mare, who loved Kelsier instead of Marsh.
Throughout the series, Marsh mentions how he wishes he could use Allomancy like Mare and Kelsier could.
Appearance in The Alloy of LawEdit
In Sanderson's fourth addition to the Mistborn series, The Alloy of Law, over 300 years have passed and characters from the original three books have sunk into legend and even become religious figures. Though the Steel Inquisitors are no longer remembered, the personification of Death has come to be represented by a man with "Iron spikes for eyes." In the Epilogue, Marasi, one of the main characters, is confronted by a man whom she assumes to be this "Ironeyes." Though he does not directly state his identity there are two quotes that strongly suggest that he is Marsh: "...With emotional Allomancy. I sometimes Pull too hard. I've never been as good at this sort of thing as Breeze was..." "That one does not like unanswered questions, but he does my brother's work, and that is something I feel inclined to encourage...." In addition, he is described as having a crushed eye socket, matching an injury that Marsh sustained during Well of Ascension.
Marsh is also noted, in book one (Mistborn), as having previously been refered to as "ironeyes", by the crew, due to his steely and un-nerving gaze. The nickname was abandoned, because of the unsettling similarity between this name and the actual appearence of the Steel Inquisitors--thus adding to the later Irony of Marsh eventual becoming not only the last surviving Steel Inquisitor, but also being known solely the nickname "Iron Eyes"--a name that at that point in time had become loosely synonymous with "Death".