As seen on [url=http://img139.imageshack.us/img139/5056/floodguts.png]Bungie's Christmas card[/url] the Rookie and N6 are the exact same size, so it is obvious that they are the same person, as already hinted at extensively in both ODST and Reach.
[b]Note: I've now messaged a forum ninja to close this, post at your own discretion.[/b]
[Edited on 12.14.2010 6:03 PM PST]
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Simple. ONI has many secrets. Noble Six, wounded but alive, managed to find a usable transport amongst the wreckage and fled the dying planet to Earth. Arriving after The Covenant were already on the way, ONI reassigned him to an ODST squad due to his substantial injuries and the fact that most of the other Spartans were dead and he is assigned to Sgt. Buck's squad, where he is silent due to the horrific memories of Reach.