The author of the Book of Jude.
Jude 1:1Jude, a servant of Jesus Christ and a brother of James, To those who are called, beloved in God the Father and kept for Jesus Christ:
Here, Protestants refer to him as the physical brother of Jesus Christ, while Catholics refer to him as a very close disciple of Jesus Christ. (In Catholicism, the perpetual virginity of the Virgin Mary is a doctrine.)