Quiet and withdrawn, Nagisa is the Child of Water, the guardian of the Cup of Life, and the Keeper of the Lighthouse of Cabra. After helping to save Amber during the Annadil Crisis, he then devoted himself to the study of water magics, the maintenance of the Lighthouse of Cabra, which provided the only safe route past the Rebman Maelstrom, and to the search for the Demi-Jewel of Water during the interregnum between the Annadil and Herald Crises.
During this time, he also secretly married Princess Gabrielle of Ember and they had a child together, Princess Shinobu Minor.
Fate saw fit to deprive him of a role in solving either the problem of the Maelstrom or the problem of the missing Demi-Jewel. While it remains unclear as to why the Rebman Maelstrom came to an end, Prince Lucien found and claimed the Demi-Jewel of Water for himself. After Lucien's death at the Battle of Rebma on the Day of the Dead, Nagisa claimed the Demi-Jewel for himself and remains its keeper.
During the Herald Crisis, he helped fix Morgenstern after Morgenstern's fit of insanity, had a very public fight with Prince Lucien at Marching, rescued Prince Lucien from Bloc Rebmaquois despite that, and nearly died at the hands of Prince Saionji. At the Day of the Dead, he had to fight Hel with the Demi-Jewel of Water, and nearly died. He has since become the keeper of the Demi-Jewel of Water.
Nagisa is rarely seen at Amber's court, but does stay in touch with Miki and Kozue (his parents), Madoka (his step mother and sometimes musical partner), Shinobu Minor (his daughter), and Shinobu Major and Naru (his half-sister and neice respectively). He is on fairly good terms with all of them, although he's often embarrased by Kozue, Shinobu Major, and Naru's fits of temper, or Kozue's mercurial nature. He also is on very good terms with Lady Yumi. He seems to be on good terms with Kaname, and to view her friends Melissa, Kurz, and Sousuke with favor. He's somewhat afraid of Baron Munchausen, and is known to intensely dislike Asuka.
He has recently helped fix the aftermath of the Hemingway battle, and is now headed into Shadow with Haru and Naru to go hunt down any plots of his mother which may be about to surface.