... and, no, not Captain America thanks!
This is for WWWII really but the concept holds good for any war.
The idea is that this guy turns up in the midst of a frightened unit as lone soldier from another unit who have been in action for some time, acts as a kind of calming avuncular veteran-type, has some useful local knowledge and some superior skills (maybe he's a good shot or good at sneak attacks on sentries or whatever, somewhat in the manner of Sgt Rock I suppose); however he'll disappear before the end of the action and when the unit tries to find out about the unit he's from, they'll be told that no such unit existed or they were wiped out in WWI or that everyone in the unit was killed a month ago in a failed mission to do whatever this unit succeeded in doing or whatever.
So he's kind of a helpful ghost or psychic manifestation of the need for help in a desperate situation.
Any legends of ghost units, ghost veterans, or whatever that I can draw on here? Or a generic kinda template/stereotype I can use to name him (like 'Tommy Atkins' in the British army?). Is 'GI Joe' the term I'm looking for? Or is it Corporal Kowalski (with his little dog)?
Also I have no idea if he's Army or Marine Corps...
And yes, the concept is totally ripped from that Stan Ridgeway song 'Camoflage'.
Thanks for the musings of your collective gargantubrains, chaps and chapesses.