Which characters in Henry V would be considered socially marginal?
I would say that the character of Fluellen could be considered "socially marginal." Although he is a captain in the Welsh army, a man of "much care and valor" he is also called "out of fashion." Additionally, Fluellen undermines himself by acting like a know it all, in his attempts to sound intelligent; he often he bungles words ("falourous" instead of "valourous" for example).
He also does not endear himself to superiors by claiming to have military expertise he does not possesses, as in this speech from 3.3.36-42:
Captain MacMorris, I beseech you know, will you
vouchsafe me, look you, a few disputations with you, as partly
touching or concerning the disciplines of the war, the Roman
wars, in the way of argument, look you, and friendly communication: Partly to satisfy my opinion and partly for the satisfaction, look you, of my mind. As touching the direction of the
military discipline, that is the point.