Who is Pentheus and what role does in play in the play?

Pentheus

Young boy, king of Thebes. We are told later in the play that Pentheus does not yet have a full beard; that puts his age at no older than his late teens. He is an impatient and impetuous young king, and he rejects the new religion and its god. Some have argued that he symbolizes rationality, but close attention to the play reveals that he is anything but rational. He rejects anything he does not understand, and his greatest crime is his lack of self-knowledge. Dionysus preys easily on the boy's weaknesses, turning the young king into a scapegoat for the crimes of Thebes. But Pentheus' excesses and crimes against the god are understandable. He is only human, and he is only a boy. For these reasons, Dionysus' revenge is brutal and excessive.

Source: