What is the background of the nesoi of how she became a goddess?

The Nesoi (Greek Nῆσοι "islands"), in ancient Greek religion, were the goddesses of islands. Each island was said to have its own personification. They were classified as one of the Protogenoi, otherwise known as ancient elemental Greek primordial deities. The Nesoi were thought to have been Ourea who were cast under the sea during one of Poseidon's rages.