Tu was a creature that appeared in a fragmentary version of Tolkien's working notes for the Coming of Men, given in the Book of Lost Tales as "Gilfanon's Tale." Tolkien apparently decided not to incorporate the tale into his mythos, as it did not survive to become part of the chapter of The Silmarillion entitled "Of the Coming of Men into the West."
Gilfanon's Tale is only a page or so long, with notes as to what happened afterwards. Tu is described as a "wizard" or "fay" who ruled the Dark-Elves of the further east of Middle-earth around Cuievenan just before the making of the Sun. One of his subjects, Nuin Father of Speech, came to a hidden valley of great loveliness and found there sleeping figures young but not children, boys and girls: the Fathers of Men. He told this to Tu, who fell into great fear of the One, and told Nuin that these were the Fathers of Men, who were to be undisturbed. Notes then tell that Nuin awoke two of the Sleepers, Ermon and Elmir, and taught them speech; that Tu and Nuin shepherded the Sleepers when they awoke with the Sun, and that Men split into factions in the years afterward. Tu grew to shun day and took to dwelling underground. A demon named Fankil, son of Melkor, arose and set Men at each other's throats, and especially against the Elves. The folk of Ermon stood with the Elves, but Tu came forth and destroyed Fankil, and was buried under falling hills.
What manner of being Tu was, shall doubtless never be known, other than that he was no Elf, nor Wizard, nor any of the incarnate Maiar, but a new thing entirely, even as Bombadil and Goldberry doubtless are. He was described a great magician and a being of power, benevolent but testy, and afterwards came to have great hatred of Men for the deeds they did to his Elves.
His role later became Thu, who was later Sauron.
His role has been expanded further in James Farrell's version of the Book of Lost Tales: