He'd probably be shocked. Harry has always seen Draco as a bit of a bully, and the fanfic might show a different side of Draco that he wasn't expecting.
He might be initially suspicious. Draco is used to a world of rivalries and power plays. When he meets Harry Potter's sister, he may suspect she has some hidden agenda or is just another Gryffindor trying to undermine him.
The Malfoy family would surely be enraged. Lucius Malfoy would use all his influence and power to get Draco back. This could lead to a conflict between the Malfoy family and Harry's supporters, causing a rift in the wizarding community.
Draco would probably be extremely angry. He might feel a deep sense of hurt and betrayal, especially if he considered Harry a friend or at least an acquaintance he could trust. He could start plotting revenge against Harry, using his own cunning and Slytherin skills to make Harry's life difficult at Hogwarts.
Harry might initially be very angry and hurt. He could cut off all communication with Draco and become withdrawn. In some fanfics, he would even start to avoid places where he might run into Draco.
Harry might be confused at first. He doesn't understand why Draco would think he's a doll fan. So he might just shrug it off and not pay much attention, thinking Draco is just being his usual difficult self.
Draco might become withdrawn. He could isolate himself from his friends and spend more time alone in the Slytherin common room, brooding over what Harry has done to him.
In many fanfics, Draco's reaction turns into a sense of protectiveness. He feels a need to shield Harry from any further harm, whether it's from the people who abused him in the past or from the judgment of others. For example, in some stories, he'll stand up to those who make snide remarks about Harry's past, using his Slytherin cunning to put them in their place. He also tries to be a source of comfort for Harry, which is a big change from their initially antagonistic relationship.
He could also react with suspicion. Draco might think that Harry has some ulterior motive, especially if the confession is of a more positive nature like friendship or love. He would question Harry's sincerity and might even test him in some way.
Often, Harry reacts with anger. He's not one to take being cheated on lightly, whether it's in a relationship or in a competition. His Gryffindor pride kicks in, and he'll want to confront Draco immediately.