6 persons entered a library the day a book got stolen. Each of them entered the library once and only once, and stayed there for some time, then left. If two persons were in the library at the same time, at least one of the two saw the other. After the investigation, the testimonies were

• Albert stated he saw Bernard and Édouard
• Bernard said he saw Albert and Isabelle
• Charlotte said she saw Didier and Isabelle
• Didier said he saw Albert and Isabelle
• Édouard said he saw Bernard and Charlotte
• Isabelle said she saw Charlotte and Édouard

Only the culprit is lying. Who is he/she ?