Mission to Ammonihah
The first introduction to Amulek is during Alma's mission to Ammonihah covered in Alma 8-15. Amulek, a resident of Ammonihah, was originally directed to help Alma by an angel in a vision. It appears that he was spiritually humble, but surprised by his calling. He seemed to consider himself mediocre as far as religion was concerned, but he seemed to be a man of some stature in the city. He was taught by Alma for some days before they proceeded to preach to the inhabitants of the city of Ammonihah.
In Ammonihah, Amulek startled the people in that he presented a second witness to the words of Alma, whom they had discredited for being alone in his testimony. He was also instrumental in the convincing of Zeezrom, a chief opponent to the gospel, of the truth of Alma's words.
Alma and Amulek were both imprisoned, then forced to watch the burning of women and children. They finally escaped the city when, by the power of the Lord, the ground shook and shattered the prison walls.
Further Missionary Work
After the mission to Ammonihah, Amulek continued with Alma back to the city of Zarahemla.
In Alma 16:13,15 it is recorded that Amulek, together with Alma, helped to build up the church in Zarahemla and further in the land round about.
In Alma 31:6 Amulek is mentioned as being selected by Alma to be part of a missionary effort to reclaim the apostate Zoramites. A powerful sermon of his, delivered during this mission, is recorded in Alma 34
LDS scholars refer to Amulek as a clue that other peoples inhabited the "land of promise" when the Nephites arrived. Amulek tells Alma that he is a Nephite. Were there no other peoples, that sort of introduction would be unnecessary, since all the inhabitants would have been either Nephites or Lamanites and Lamanites were distinguishable by their appearance.