It is not clear why the name of the brother of Jared does not appear in the Book of Mormon. However, the following are possible reasons: (1) He may have omitted his name out of modesty (John the Beloved did essentially this same thing in the Gospel of John, which he wrote). (2) The book of Ether is clearly a family record of Jared, not of the brother of Jared; Ether—the final writer and perhaps the abridger of the record—was a descendant of Jared and might naturally have emphasized the achievements of his direct ancestor rather than the brother of his ancestor. (3) Moroni may have omitted the name in his abridgment because of difficulty in translating (or transliterating) the name into the Nephite language.
Although the actual name of the brother of Jared is not mentioned in the scriptures, many Latter-day Saints quote the following story related by George Reynolds to indicate that his name was revealed to the Prophet Joseph Smith after the translation of the Book of Mormon:
While residing in Kirtland Elder Reynolds Cahoon had a son born to him. One day when President Joseph Smith was passing his door he called the Prophet in and asked him to bless and name the baby. Joseph did so and gave the boy the name of Mahonri Moriancumer. When he had finished the blessing he laid the child on the bed, and turning to Elder Cahoon he said, the name I have given your son is the name of the brother of Jared; the Lord has just shown (or revealed) it to me. Elder William F. Cahoon, who was standing near, heard the Prophet make this statement to his father; and this was the first time the name of the brother of Jared was known in the Church in this dispensation. (“The Jaredites,” Juvenile Instructor, 27:282. Also Improvement Era, 8:705.)
In connection with this name, it is interesting to note that the major encampment of the Jaredites on the shore of “that great sea which divideth the lands” was called “Moriancumer.” (Ether 2:13.)