Ephraim in the Book of Mormon is not an individual but the tribe descended from Ephraim, son of Joseph. The tribe became the main population of the northern kingdom of Israel, set against the southern kingdom of Judah. Every reference falls within Nephi’s quotations of Isaiah, except for a place name in the Jaredite record.
Isaiah names Ephraim allied with Syria against Judah: “Syria is confederate with Ephraim” (2 Nephi 17:2), and Syria, Ephraim, and the son of Remaliah took counsel together against Judah (2 Nephi 17:5). Ephraim was foretold to be broken as a people within sixty-five years (2 Nephi 17:8), its head being Samaria (2 Nephi 17:9). A following verse records Ephraim and the inhabitants of Samaria speaking “in the pride and stoutness of heart” — their boast, not a reception of the Lord’s word (2 Nephi 19:9). Manasseh and Ephraim devoured each other yet together stood against Judah (2 Nephi 19:21), this hostility dating from “the day that Ephraim departed from Judah” (2 Nephi 17:17). A later passage foretells that the envy between the two would cease, with neither Ephraim envying Judah nor Judah vexing Ephraim (2 Nephi 21:13).
In a Jaredite account, the hill Ephraim is where Shule mined and smelted ore to make steel swords for his followers before regaining the kingdom for his father Kib (Ether 7:9). In modern revelation the Book of Mormon is called “the stick of Ephraim,” the record whose keys were committed to Moroni (Doctrine and Covenants 27:5).