Metis is the Titaness fly goddess of wisdom and judgement that wed Zeus, the King of the gods and the ruler of Mount Olympus. Once wed, Zeus tricked her to fly into his head and gave birth to Athena, the goddess of wisdom, as she sprung full-grown out of Zeus' head, despite being a Virgin goddess. Although, she is the original mother of Athena.
They were wed and Metis became pregnant with Athena. When this happened, Zeus heard a prophecy that his son would over throw him, like he did to his own father. So Zeus swallowed Metis alive, with Athena inside her. Thus, this ended their marriage.
Metis is known as a fly goddess, though isn't really a fly. She apparently shrunk down to the size of a fly and went into Zeus' ear. Her buzzing is also known as her talking, but it is too high-pitched to hear and only Zeus can hear her speak, while the others hear something buzzing.
Metis starred a recurring role in the series from books one to eight, however she and Zeus split up, making her fly out of his head and never get mentioned again, erasing her from the series.
Athena - As her real mother, Metis did care for Athena. It is unknown where she can be now.
Zeus - Formerly her husband, Zeus and Metis tend to get along sometimes.
Metis' official Greek name is Μητις.