Harry Potter loves to read; Fleur Delacour loves to read; ergo, they form a book club. The Fleur and Harry Book Club, to be precise. At first, it's just the two co-founders, trading book titles and discussing them at the end of the week, pointing out various plot points, plot holes, likes, dislikes, the whole nine yards. Fleur is very happy with the arrangement because being close to Harry has always made her day that much better, that much brighter. Fleur has never been happier, spending time with the two most important, non-family, things in her life; Harry Potter and books.
Then, one day, she walks into the room and finds a woman (whose name is definitely not on the club roster) chatting with Harry, laughing and smiling at him, casually touching his arm, sitting way too close to Harry, looking like she's having the time of her life. Suddenly, this woman has casually inserted herself into her Harry time, her time to unwind, relax, and enjoy the uniquely wonderful company of Harry Potter.
Fleur Delacour doesn't like this; she doesn't like this one bit.
P.S. The woman can be whoever the author wants really, an OC or a canon character, just has to be someone who Fleur clearly thinks is encroaching on her, not yet but totally will be when she figures out how to ask him out, man.