There are no Harry Potter characters in the WWoHP. The closet thing to characters are the Frog Choir and the Tri-wizard dancers who perform and do pose for some photographs after their performances. However, these singers/dancers are not "official characters" such as Fleur or Krum, they are just random "students". If you're looking for characters such as Harry, Ron, Hagrid, Dumbledore ect you will not find them in the park at all in person as JKR does not allow it.
However, I'm sure if he would like some autographs from Hogsmeade that many of the witches and wizards working in the village as well the Hogwarts Express conductor would love to sign his book for him.