Who will it be? When will he be here? Let's see what the great Maimonides (1135-1204) had to say about it in his Mishnah Torah, Laws of Kings, Chapter 12, Law 2:

"All these matters concerning the Messianic times cannot be known by man until they actually occur, for they are mysterious and undefined in the words of the prophets. Even the sages have no clear established tradition regarding these matters beyond what is implied by these verses. Hence, there is a divergence of opinion among them. In any case, neither the sequence of these events nor their precise details are among the fundamental principles of faith. One should not occupy himself at length with aggadoth and midrashim that deal with these and similar matters, nor should he deem them of prime importance, for they bring one to neither the awe nor the love of God."

"We cannot know"