: I don't know, he's the Rookie; they know practically nothing about him. And
: he's only the "Rookie" because he's new to their team, not
: because he's a new ODST. I don't think it's out of the question to assume
: they haven't seen him outside of "work." And if he is indeed an
: ONI operative, he's obviously going to be keeping his secret
: background...well, secret. That said, he was apparently hand picked by
: Buck. But if Dare/ONI wanted him on the mission, she could have set Buck
: up to pick him.

From the pool of thousands in the Helljumper division, if the Gunny didn't trust him to mesh absolutely perfectly with his band of merry men he wouldn't have been chosen. Had he been planted by Captain Dare in the manner which you suggest, the Rookie was still subordinate entirely to Buck as a lance corporal. That's rather insulting for a presumably 81+ Marine who's talents could be implemented independently of this small band. In fact, ONI would've been wise to have him in command of this mission instead of Buck if ORION-047 was the Rookie.

I think you're too brazenly connecting shadowy figure with super shadowy figure. One is someone who we've seen in action, who's actions and body language doesn't fit anyone but a young, but seasoned Marine. The other exists solely as a little extra bit in Halsey's journal, presumably as an ego-boosting token to the advancement in augmentation procedures from ORION to SPARTAN.

If Rookie were to be anything "special", it's more likely that he's the child of an augmented human, or perhaps someone who was transferred out of the Three's Alpha Company, or more likely, Beta Company. Had he been in Beta, Rookie would've been born approximately in 2530, inducted into the program in '36, and discharged sometime prior to or after Operation TORPEDO in '45. This would have made him age 17 in '47, when he enlisted in the Marine Corps, thus a deterrent against mass eyebrow raising.

Then again, why does every badass in the Haloverse need to be a supersoldier? Humans are granted remarkable daring and strength from birth and I don't think these stories lend much credibility to that truthiness.