Ancient human. It's funny, everyone refused to believe it was human (even though it clearly was) and thought it was a dumb interpretation of Forerunner structures. In response to that, Frankie said, "Who said it was Forerunner?"

Ah. Finally solved that mystery. How does one fit in the prototype suit. It looks to big to have limbs going through the suits limbs, but when Ghost gets his arm blown off, there is large amounts of blood, like his actual arm got destroyed