Most people know about Radha, who is said to be Krishna's lover and his favorite among the Gopis, whom Krishna spent time with during his childhood in Vrindavan and Gokulam. She's revered by many Vaishnavas, particularly those of the Gaudiya and Nimbarka Sampradayams. Yet the Sri Vaishnava Sampradayam (of which I'm a member) doesn't emphasize Radha. In part that's because Sri Vaishnavas focus on the Vishnu Purana and the Srimad Bhagavatam, neither of which even mention Radha's name. Yet Sri Vaishnavas do recognize a figure similar to Radha. Let me explain.
The Alwars (also spelled Azhwars) are a group of 12 ancient Vaishnava saints who lived in Tamil Nadu and are famous for their poetry in praise of Vishnu. The collection of their 4000 poems, known as the Naalayira Divya Prabhandam, is considered by many to be the "Dravida Veda", or South Indian Veda. The Alwars are crucially important figures in the history of Vaishnavism; it is the beliefs and principles embodied in the Alwars' poems that ultimately gave rise to the Sri Vaishnava sect.
In any case, the Alwars discuss a figure named Nappinnai, daughter of Yashoda's brother Kumbagan. It is said that Krishna married Nappinai after defeating the seven bulls of Kumbagan. Sri Vaishnavas believe that Nappinnai is an incarnation of Vishnu's wife Nila Devi. Most people only know about Vishnu's wives Sri Devi, aka Lakshmi, and Bhu Devi, aka Bhumidevi, but according to Sri Vaishnavism Vishnu has a third wife named Niladevi; the Nila Sukta of the Yajur Veda is addressed to her:
O thou of the three-and-thirtyfold Stoma, lady of the world, Breathed on by Vivasvant, do thou be gracious to us; Rich in ghee, O Savitr, through thy overlordship, Be the bounteous region rich in milk, for us. The firm among the quarters, lady of Visnu, the mild, Ruling over this strength, the desirable, Brhaspati, Matarisvan, Vayu, The winds blowing together be gracious to us. Prop of the sky, supporter of the earth, Ruling this world, lady of Visnu, All-extending, seeking food, with prosperity, May Aditi be auspicious to us in her life.
In any case, my question is, apart from the Alwars' poems, are there any scriptures which discuss the story of Nappinnai?
Now this chapter of the Srimad Bhagavatam describes how Krishna wins the hand of his queen Satya after defeating the seven bulls of Satya's father king Nagnajit (not to be confused with this king Nagnajit). This is exactly how Krishna won the hand of Nappinnai. So does this have some relation to the story of Napinnai?
Also, I've heard that Yashoda's brother Kumbagan is mentioned in the Harivamsa. Does anyone know if that's correct?