Roland Weary is an unpleasant little jerkface who finds Billy wandering around behind enemy lines after the Battle of the Bulge in Luxembourg. He is an 18-year-old American bully who thinks it's cool that his father collects medieval torture implements. He's basically Slaughterhouse-Five's version of Joffrey Baratheon.
Weary is incredibly well-equipped with everything you could possibly need for war—good boots, sharp implements, lots of weapons—and he likes to fantasize about what a great soldier he is. In reality, he's a violent little creep who fires one shot during his first battle, which alerts the Germans to where his machine gun is. As a result, the Germans kill Weary's entire gunner detail.
Weary won't leave Billy behind, not because he likes Billy or is compassionate or anything. Instead, Weary is busy creating a story in his head about the war. In it, he and two other scouts manage to drag Billy to safety even though Billy is totally unequipped for battle. Weary calls his trio "The Three Musketeers." As he drags Billy across the Luxembourg countryside, he fantasizes so hard about how he and these scouts will be decorated with medals for saving Billy that he completely loses track of where he is.
When the two scouts lose patience and leave Weary and Billy behind, Weary blames Billy. When the Germans come, Weary is pointing his gun at Billy. The Germans cannot understand why one American would be trying to shoot another on German territory. (This irony of pointless violence in the midst of much larger historical events repeats itself with Paul Lazzaro, by the way.)
Weary winds up dying of gangrene because his feet are too damaged by a pair of wooden clogs the Germans make him wear in exchange for his own state-of-the-art combat boots. He dies cursing Billy's name. Weary is as inexperienced as Billy himself, but he has grand ideals about war and its glories. He believes in and thrives on violence, thus making an already awful situation even worse for everyone around him.