Marsh is Kelsier's brother. He is a Seeker.
History
As part of a plot to dethrone the Lord Ruler, Marsh became a spy and acted as an Obligator, providing valuable information to Kelsier's crew, including the information about 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.
Personality
Marsh is, by all accounts, a serious and no nonsense man. He is Kelsier's older brother, and frowned upon Kelsier's criminal activities.
Marsh's personality was greatly influenced by a few events. One is that he worked as leader of the Skaa Revolution for many years before surrendering the post to Yeden.
Another thing about Marsh is that he once 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 along with Mare and Kelsier