Her Smile.
Very few things in life could make Fleur Delacour smile. Not the smiles she gives out in public that if you look carefully or know Fleur very well seem fake and forced. A true genuine smile from Fleur Delacour made her shine in a different way, making her incredible Veela enhanced beauty seem less cold, distant and cartoonish and make her warmer and ever more beautiful to behold. Among the few things and people can achieve this would Fleur's little sister Gabrielle who never fails to make her older sister smile.
And when it is time for the Triwizard Tournament, Fleur is selected to be among the Beauxbatons delegation for the chance to be selected as school champion. When Fleur arrives at Hogwarts, she entraces many male (and some female) students from Hogwarts and Durmstrang. But not a single one of them can make Fleur truly and genuinely smile. That is of course, Fleur meets the Boy Who Lived Harry James Potter.
Having heard all the stories and rumours of Harry Potter, Fleur immediately assumes that Harry is nothing more than an attention seeker and gloryhound with an ego that would dwarf the planet Saturn itself. But to her surprise, Harry is nothing like the stories and rumours about him.
And he is able to make her smile.