Who is Pylades, Orestes' Nurse from The Oresteia and what is their importance?

Pylades is Orestes' friend and traveling companion. He encourages Orestes to kill his mother, and as a result, his character is seen as the embodiment of the dangers of giving in to the desire for revenge. If Pylades hadn't said what he said at the moment he said it, Orestes might not have killed his mother, might not have been driven to near madness by guilt and remorse. Also, he might not have ended up living out the rest of his life in exile (his future as defined in the final moments of The Eumenides).

The Nurse only appears in one scene. She does, however, serve two functions. First, she is a positive example of maternal affection..... the kind of affection that Clytemnestra is incapable of. Second, her character is used by Orestes to lure Aegisthus into what he thinks is safety.