Junior isn't the most individualized of characters, and in a lot of ways he's a type—a representative for something else: the platoon's African American members.
Junior only appears in a handful of scenes, but he's usually talking about politics. Thus while Taylor, Elias, Rhah, Harold, King, and others are smoking pot, Junior is elsewhere, telling Bunny about about white people have been "trying to keep the black man down and string him out on that s*** [marijuana]."
While Junior talks about the politics of black and white in a way that highlights the racial divisions of the 1960's, and of the platoon, he's also kind of a coward. He falls asleep on ambush, and tries to blame Taylor.
And near the end, he puts an excess of insect repellant on his feet in order to make them appear so swollen he can leave the field of battle. He ultimately dies for being cowardly. Fleeing his foxhole in the final battle, he runs into a tree, getting knocked out before ultimately killed.