Abzu, also known as the "God of Darkness", was some sort of a supreme being who created all universe's darkness, and his seven elements (water, fire, earth, wind, thunder, light and shadow), therefore, he is a primordial god. However, the Earth was dominated by the Olympian gods, including Zeus, who gave the planet for his daughter, Athena; thus, the Earth's Cosmo probably did not use his 7 elements.
But, Medea invoked a meteor that brought the soul of Abzu to Earth, along with two babies, each one representative of the two dominant elements: Light and Shadow; the presence of Abzu on the planet, made the Cosmo to use his seven elements.
These babies were Aria, representing the light, and was created by Mars (to do things your cosmos of darkness don't), and Kouga, who was supposed to be possessed by the god of darkness at the time, but the light cosmos of Athena saved him. Abzu was sleeping in Kouga, now with a light cosmo borrowed by Athena.
The purpose of Abzu (spread darkness in the world) did Medea manipulates his host, and manipulate him later. After Medea enters Kouga darkness, the light of Athena went off inside him, and so absurd awake. After much effort, Seiya and Yuna expel Abzu from Kouga, and Kouga comes into his darkness (where he held Athena) and defeat his original body.