The Sun Queen is the queen of the sun.
Fire is Sun Queen's hero, because he saved her from being defeated by the Dark Queen.
Think of it this way:
Dark Moon vs The Sun World
Fire Kingdom vs Dark Fire Jail
Fire Queen and Sun Queen are good friends.
Since Sun people are a race of Fire people, Sun and Fire people have same powers.
All except Light Blast and Fire Blast.
Fire and Sun QueenEdit
Fire and Sun Queen are Best Friends.
Fire protects her.
While Sun Queen protects the people.
They a great team.
Fire even insisted that she join TFH(The Four Heroes)
But she said no because she has duties to take care of as queen.