Isaiah was one of the twelve disciples Jesus chose during his ministry among the Nephites, named in the list alongside Nephi, his brother Timothy, Timothy’s son Jonas, Mathoni, Mathonihah, Kumen, Kumenonhi, Jeremiah, Shemnon, a second Jonas, and Zedekiah (3 Nephi 19:4). He is distinct from the Old Testament prophet Isaiah, whose words the Nephite record quotes elsewhere.
With the rest of the twelve, Isaiah taught the assembled multitude, prayed to the Father in the name of Jesus, and was baptized by Nephi, after which the Holy Ghost fell upon the disciples (3 Nephi 19:5-13). The record gives no further detail about him individually.