Nagi is the Grand Duke of Artai and Nina's master.

In the anime version:

He enjoys teasing Mashiro and seems to know exactly what buttons to press to get his way. Upon his first meeting with Mashiro, he asked her if she was the "fake princess of Windbloom." Since then, the two have had a love-hate relationship. He's even had proposed marriage to Mashiro and nearly kissed her except for Arika's interference.

In the manga version:

In the manga version, Nagi is truly in love for Mashiro, not knowing that his beloved princess is actually a guy.

He is used as scapegoat by the real villain who wants to kill Mashiro-boy and blame Nagi.