At the end of the story, Natsu makes a difficult choice to go against Lucy’s wish for her sake. Even though Natsu knows that he would also end up carrying a heavy burden because of this, he makes that choice so as to protect Lucy. I think Natsu’s feelings for Lucy might go a little beyond camaraderie and friendship.
Natsu’s feelings for Lucy have yet to be clearly depicted in the original work, but I had Mashima-sensei’s consent to develop it a little in the movie. I tried to express Natsu’s feelings for Lucy, which go a little beyond friendship, through his gaze and movement.