In addition to Enlil and Inanna, Ninurta was also sacred to Nippur. His temple, the Eshumesha, was one of Nippur's largest. However, as the multifaceted, contradictory Inanna arose from foreign and local gods, and as Enlil's identity was riddled with ambiguity, so was Ninurta.
He was known as Enlil's first-born, but the story of Enlil and Ninlil asserted that Nanna had been their first child. Further, Ninurta had conflicting domains, as both a fertile agrarian and a fickle, destructive rain god.
Ninurta as Fertility God
Instructions of Ninurta was a text dealing with barley cultivation; and a Sumerian hymn called him 'the farmer of Enlil' whose 'life-giving semen' filled the canals and let barley proliferate.
Ninurta as Warlike Weather God
The Old Babylonian text The Return of Ninurta, copied through many eras, portrays a warlike Ninurta (Leick 2001, p 155-156). It relates how he did battle in the kur, referring to the troublesome mountains to the north and east of the Mesopotamian plains. Ninurta, storm of the rebellious lands, swept like a flood, destroying the fortresses of the rebellious lands. Victorious, he drove his booty-laden chariot back to Nippur. In full battle regalia, surrounded by his captains, he prepares to enter his father Enlil's temple, the Ekur. Even Enlil is frightened in his residence. Ninurta then enters the Ekur and lays out the plunder, impressing all the gods including his mother and father.
The gods are worried about his excessive strength, perhaps even great enough to overtake Enlil, but flattery seems enough to placate him. Ninurta begins a hymn-like passage of self-praise which takes up much of the text. He sings about his trophies, weapons and strength. He is asked to humble himself and remain pacific against his city, and he returns to his own temple where his consort awaits. This may articulate how a city's inhabitants must have felt a mix of celebration, pride and caution when their king's fully armed triumphant army made its return.
Tension with Enki
Enki was not the supreme deity, but was nonetheless revered throughout Mesopotamian as the god of wisdom and learning. However, as Enlil's worship expanded, it began to grow at the expense of Enki's worship. This may explain why priests loyal to Enki wrote disparaging literature against Ninurta. Not only was Ninurta the son of Enlil, but he was an amplified, over-the-top version of Enlil.
In King, the Splendor of his Storm is Overwhelming (lugal-e ud me-lam-bi nir-gal), Ninurta goes to Eridu and is belligerent and demanding towards Enki. Enki's response is lost, but Ninurta returns to Nippur satisfied. In another text, Ninurta has entrusted the Tablets of Destiny to the young of the Anzu-Bird, but the tablets have fallen into the abzu (the domain of Enki). Ninurta insists Enki return the tablets and even attacks Enki's vizier, and Enki fashions a giant turtle out of the abzu-clay which bites Ninurta's toes. Enki then digs a pit into which both Ninurta and the turtle fall. Ninurta's mother Ninlil begs for Enki's mercy. He eventually relents, telling Ninlil that now she owes him a favor.
Leick 2001, p 155-156