Rosencrantz and Guildenstern seem incapable of functioning independently, so they're basically one character, no matter what they might say. The technical term to describe the two of them would be "sleaze-balls," or the ever-popular, "slimy sellouts." Rosencrantz and Guildenstern show up in Denmark to serve as paid informants on their friend from college, and they practically fall all over each other in their attempt to suck up to King Claudius. Luckily for our amusement, they're as incompetent as they are dishonest; Hamlet sees right through them, and they make good targets for his mockery.
It does seem a little harsh for Hamlet to send them off to die, though, as Horatio points out. These two characters function well as reminders of Hamlet's weird decision-making process. He hesitates (understatement) to kill Claudius, who arguably deserves it, but doesn't flinch in exterminating his own two friends who, let's face it, were probably just college students hard up for cash.
Even though Rosencrantz and Guildenstern die before the mass death scene in Act V, Shakespeare works it so that we find out they've been killed at the same time everyone else is dying. A British ambassador shows up in the final scene for the sole purpose of saying, "Rosencrantz and Guildenstern are dead" (5.2.355). Contemporary playwright Tom Stoppard liked this line so much that he wrote a play from the perspective of the two characters and titled it Rosencrantz and Guildenstern Are Dead.