Maya is Kai and Nya's mother, the wife of Ray, and the previous Elemental Master of Water. She was seen in a flashback in "Spellbound" . It was confirmed at the 2015 Comic Con that she was indeed Kai and Nya's mother. 
The Tournament of Elements
Maya appeared in flashbacks during this season, fighting alongside the other Elemental Masters against the Serpentine.
In this season it was revealed that Maya was Nya's mother, thus making Nya the heir to her powers with the potential to become the Ninja of Water.
At the conclusion of this graphic novel, Sensei Wu has a vision of Maya and her husband.
In this special Kai and Nya celebrate the memory of their parents at their shop.
Hands of Time
In this season Maya's history is fully explored, and it is learned that she took part in not only the Serpentine Wars but in the battles against the traitorous Elemental Masters Acronix and Krux, the Hands of Time. Maya and her husband, both skilled blacksmiths, forged blades out of Chronosteel to drain the powers of Acronix and Krux; the process left them both exhausted and unable to take part in the final battle. Afterwards, the pair were entrusted with the first of the four Time Blades to return to Ninjago, and hid it in the Boiling Sea in the sunken library of Hono Mizu. They would then have Kai and Nya and be befriended by the kindly Dr. Saunders, only for him to reveal himself as Krux and demand that they work for him in exchange for his promise not to harm their children.