The Janakas were a lineage of sage-kings of Mithila or Videha in ancient northern India. The best known was Seeradhwaja Janaka, more popularly known as Raja Janaka or King Janaka. He is mentioned in the Ramayana as the father of Sita. The Ashtavakra Gita is a dialogue between the sage-guru, Ashtavakra and his disciple, King Janaka.
H.H. Shri Mataji Nirmala Devi has stated that Raja Janaka was the first of the ten incarnations of the Primordial Master:
I will tell you a story of Raja Janaka who was a great king in India. He was called as Bidehi - means an ascetic. And he was a king and whenever he went to any ashram the saints used to get up and touch his feet. So one disciple of a guru got very angry ─ his name was Nachiketa ─ and he said, “How can you all touch his feet when he is a king and enjoying the life like a king, living like a king, wearing a crown? How can you touch his feet?” So the guru said that. “He is the Primordial Master.” To him it does not matter whether he lives in a palace or on the street. (1983-0910)
For background information on Mithila and its wisdom-kings, see chapter 4, esp pp69-71, of Geoffrey Samuel, Origins of yoga and tantra: Indic religions to the thirteenth century (Cambridge University Press, 2008)