Who is Agrippinilla from Claudius the God and His Wife Messalina and what is their importance?

Agrippinilla is Claudius' niece and strangely enough..... his fourth wife. The couple marries after Messalina's death, though their marriage is not a love match, but rather political alliance. Though Claudius is emperor, Agrippinilla rules the roost and eventually administers the poison that will take his life.