Miller, a.k.a. Pvt. Miller, is a character on COD5 who does all the shit that the sergeant won't do. More generally, a miller is someone who you make do all the dumb shit that you won't do yourself, mostly because it is life-threatening.
Sgt. Roebuck: Miller get on those MG's!
'Friend': Hey miller, get on that grenade.
'Friend 2': Yeah miller, cross the street without looking.