What was the Brother of Jared’s name? Does it ever say? Well,
it was the custom of the people of Nephi to call their lands, and their cities, and their villages, yea, even all their small villages, after the name of him who first possessed themAlma 8:7
And the Jaredites called the name of the place MoriancumerEther 2:13
So was his name Moriancumer? It’s possible. But the custom was the Nephites’ custom, not the Jaredites’. Could they have had the same custom? Probably.
How did the tradition begin?
And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.Genesis 4:17
So it’s not that improbable that they had the same tradition.
But we do know his name. While it’s not in the Book of Mormon, the Lord has told us through Joseph Smith:
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.” … This was the first time the name of the brother of Jared was known in the Church in this dispensationGeorge Reynolds, “The Jaredites,” Juvenile Instructor, 1 May 1892, p282