Who is he? Kent Nelson (usually), a man gifted with the helmet that contains the Babylonian god of wisdom and writing, Nabu. Whenever he or anyone else wears the helmet, Nabu’s generally in control of the show from that point. More recently in the comics, the title of Doctor Fate was passed onto Egyptian med student Khalid Nassour.
Why include him? Ed Boon has always said that he wanted to include more magic users in the first Injustice, and with Injustice 2, he finally has his chance. You don’t get more magical than he and Zatanna, and his various spells would make him pretty damn powerful across the board. Plus, c’mon, the dude can control fire and lightning and has telekinesis and telepathy. That would be pretty great.