Marsh is Kelsier's brother. He is a Seeker. As part of the plot to overthrow the Lord Ruler, he became an obligator and infiltrated the Steel Ministry and provided information to Kelsier's crew such as the existence of the "Soothing stations."
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 Luthandel 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.