Well, Ron might be initially terrified of Slenderman. But when his friends are in danger, he would step up. The Weasley twins could try to play pranks on Slenderman, not fully realizing the danger at first. Dumbledore, on the other hand, would be very cautious. He might observe Slenderman from a distance, trying to understand his nature before taking any action. In a 'Harry Potter and Slenderman fanfic', these interactions would create a lot of interesting scenarios.
In a fanfic, Hagrid could be one of the first to encounter Slenderman in the Forbidden Forest. Hagrid's knowledge of strange creatures might make him think he can handle Slenderman, but he quickly realizes this is a different kind of threat. Meanwhile, the students at Hogwarts might start having nightmares about Slenderman, and Harry and his friends have to figure out a way to stop these nightmares and deal with Slenderman. Neville could also play a role, perhaps using his knowledge of plants to try and create some sort of defense against Slenderman.