Nila Devi (or Neela Devi) is the third consort of Lord Vishnu, the other two being Sri Devi and Bhu Devi. In Paramapadam (Sri Vaikuntham) Sri Devi is seated to the right of the Lord, and Bhu Devi and Nila Devi to His left. The Veda sings the glory of Goddess Nila in Nila Suktam.
What Bhu Devi (Bhūmi devi, Goddess of earth) is to Varaha avatara and Sri Devi (Sita) to Rama avatara, is Neela devi to Krishna avatara. Neela devi took avatar as Nappinnai, daughter of Kumbagan (brother of Yashoda) and Krishna won Her hand after conquering the seven ferocious bulls of Her father. Nappinnai's brother is Sudama. Sri Parasara Bhattar describes Krishna, intoxicated by Her beauty, as "Neela thunga sthana giri thati suptham".
Nappinnai – Krishna cult is mainly limited to the Tamil speaking world. Nappinnai is often referred to as Radha or Rādhārānī or "Radhika" in North India.
Andal (one of the Alvars) wanted to perform devotion to her Lord Krishna like Gopikas did in Dwaparayuga. She in her Thiruppavai, wakes up Nappinai before waking up Lord Krishna. As per Srivaishnavism, saranagati (complete surrender to the Lord) is performed through His consort and in Sri Krishnaavatharam specifically it is performed through Nappinai.
Velukkudi Swamy, a proponent of Vishishtadvaita Philosophy says that Andal, singing Nachiar Tirumozhi as the daughter of Periyalvar, mentions about Rukmini. But singing as a Gopika in Thiruppavai she mentions only Nappinnai. Alvar mentions the three nachiars (consorts) as Pon mangai(Sri devi), nila mangai (Bhudevi) and pula mangai (Neela Devi) the consort of senses. It is Neela devi who keeps Bhagavan under control by her bhogam. Swami Vedanta Desika says in Daya satakam - "nisAmayatu mAm nIla yat bhOga patalai: druvam". Lord Krishna was a Yadava kshatriya varna by birth, thus he married Rukmini according to that. He was brought up as a Yadava vaishya by varna (gopa), hence he married Nappinnai according to that. Neela suuktam also says - ghruNAhi - ghrutavai - payasvati etc., mentioning the characteristics of a gopika.
The name Nappinnai is found in Divya Prabandha of Alvars and Silappadikaram.
Krishna’s wedding to Nappinnai
nandhagOpan marumagaLE nappinnaay
‘Your son is very beautiful! I can see that He will turn into a handsome youth. I wish I can promise Nappinnai to Him but I made a foolish vow that I will give her hand in marriage only to the man who can control all seven bulls tied in my barn,’ said Kumban.‘Uncle, don’t worry,’ said Krishna, ‘I will subdue those bulls this very second!’Kumban laughed at the sweet child but was terrified when Krishna went in search of the bulls. ‘Yashoda stop your son! I am afraid that He might get hurt!’ As Yashoda and Kumban rushed to the barn they found that Krishna had killed all seven bulls with one blow! ‘What a marvel!’ exclaimed Kumban.As promised he gave Nappinnai in marriage to Krishna but as they were only children, Kumban allowed Yashoda to take Nappinnai with her so that she could raise the two together. This story is mentioned by Swami Desikan in Yadhavabudhayam. http://thoughtsonsanathanadharma.blogspot.in/2013/01/thirupavai-pasuram-18.html