Cyrus the Great, founder of the great Persian Empire, appears at the very beginning of Ezra in Chapter 1. He instructs the Jews to return to their homeland and begin rebuilding their temple, giving them all the resources they need in the process. He's clearly meant to be a great and genuinely good figure—but beyond what we've just said, the book doesn't give much information about him. He's a major savior in the eyes of the Israelites. He's the one who's used to fulfill the prophecies of return.
We know from the historical record that Cyrus issued decrees allowing all displaced people in the empire to return to their homelands; he granted them a degree of autonomy and allowed them to practice their religion. The clay cylinder on which this decree was written (discovered in Babylon in 1879 and now in the British Museum) doesn't specifically mention the Jews, but they're assumed to be one of the nations. The prophet Isaiah spends a lot of time discussing Cyrus as someone given power over all the nations by the God of Israel. Writing on the cylinder, Cyrus gives the credit to the god Marduk, but no matter—this is Ezra's story and we're sticking with it.