In Sumerian mythology, Nanna (short for Nan.Nar) was the first-born son of Enlil. He was also known by the names Sin, Zu and Suen. Ninlil was his mother. He was the half-brother of Ninurta, who outranked him in the Sumerian rules of succession, because Ninurta's mother, Ninhursag, was the half-sister of Enlil. (In other words, both of Ninurta's parents were children of Anu, where only one of Nanna's parents was a child of Anu).
Nannar was the ruler of Ur. At a given moment in time, however, he was banished from the city, because he didn't respect the rules of succession and tried to usurp rulership from his brother Ninurta.
He is the father of Ereshkigal (daughter) and of the twins Inanna (daughter) and Utu (son), all of whom he had with his wife Ningal.