Rosencrantz is a childhood friend of Hamlet’s, inseparable from Guildenstern.
He knows more about the theatre than does the latter. Called to court by Claudius to spy on Hamlet, both are incompetent enough that he realizes this after a few sentences between them. Rosencrantz is a crawler before Claudius, and somewhat overdoes his compliments. He accompanies Hamlet on his ship to England, continues after Hamlet is captured by pirates, and has his head chopped off on arrival.