Tale of Two Dementors

Harry, Ron, and Hermione were about to start their fifth year at Hogwarts School of Witchcraft and Wizardry, and things couldn't be going any better. Lupin is back, and Harry gets to see his friends after a long summer. However, things are about to get complicated when a twelve year-old, golden-haired boy is chased by dementors into the opening feast. Who is he? Where is he from?