Roni
Member
Miller was a drill instructor. He participated in Solid Snake's training at FOXHOUND, that's how the two know each other.
Campbell gets Miller in as a consultant during MG2 as a "Hey, Snake, you know this guy".
You should have been able to press triangle to "Kept you waitin', Haah" just like with Kaz early in the game.
http://gtinter.msxnet.org/Setting.htm#P29
Metal Gear 2 User Manual, check Miller's bio. Focus on the word "recently" and the fact that game happens in 1999, 4 years after Big Boss deserted from FOXHOUND following Outer Heaven. There is also the wiki and the game itself, since Miller is never mentioned in Metal Gear.