Brother Kelsus was a Mechanist stationed in Soulforge Cathedral during The Metal Age. He was devoted to the Builder and believed in the Mechanist faith, but he had grown deeply troubled over Karras's actions and his complete disfavor with all things of nature, as well as his tendency to confuse himself with the Builder at times.
Kelsus believed he was not alone in his discomfort, and that others among him were not sincere in their shows of support for Karras. In preparation for the final act of Karras's plan, all Mechanists were to leave Soulforge Cathedral for one week, and Kelsus was planning to speak to other Mechanists about his concerns at the Eastport Mechanist Seminary during this week.
Before leaving Soulforge, however, Brother Kelsus was summoned directly to Karras's personal quarters and was told that he would be needed sometime during that week to operate the Guiding Beacon, and was given instructions on exactly how to operate it.
What eventually became of Kelsus is not clear, though it is likely that he was turned into a Masked Guard, since there were no human Mechanists in Soulforge Cathedral when Garrett visited, despite Kelsus being summoned to operate the guiding beacon. (T2: "Sabotage at Soulforge")