The Old Slave
We know the old slave smuggled Orestes out of town way back when, but he's also an important guiding force in the present day. He continually urges Orestes to chose action over words. In addition, he acts as the young man's spy early in the play. He will later guide Electra, along with Orestes, by urging the two of them to stop procrastinating and get to the deed. He very likely stops them from giving the game away, as he's the one to point out when Aegisthus is approaching in the final scene.