Merlin first runs into Niniane on a lake. He mistakes her for a young man who he once knew, and she takes the opportunity (and a slight glamour) to pass herself off as Ninian, Merlin's friend, and learn from him. Eventually Arthur points out her ruse, and eventually she and Merlin become lovers.  He teaches her all and passes out.  Thinking that he's dead, everyone buries him, although Ninian (now called Nimue) has terrible nightmares about how he's still alive.  However, she decides to reclaim the Grail before Morgan or Margawse can get to it.  (Margawse/Morgause is a great villain.)  She then marries Pelleas, a young king, which makes her and Merlin's reunion, after he gets free, a little uncomfortable after he escapes.  They make a clean break and Nimue takes over his roll as advisor to the king.  In the Wicked Day she appears again in that roll, a powerful woman who effectively nullifies Morgause's and Morgan's influences, but can't quite negate the damages of Guenivere, Bedivere (think Lancelot, but as he's an older character, Mary Stewart used Bedivere, one of the originals) and Mordred.  (Who she has a great scene counseling.)

Dates:
The Last Enchantment: 1979
The Wicked Day: 1983