Simplify the following text for a Grade 5 native English-speaking person.
Original: In 1991 Mulder re-opened The X-Files with Special Agent Diana Fowley , who left shortly thereafter leaving Mulder alone in The X-Files Unit .
Simplified: Fox Mulder is one of the main characters in the science-fiction show The X-Files . He is an FBI agent working in the section called the X-Files .