Ensaku Toyoshima (the manager) is the culprit.
Ran has a flashback to when she saw a younger Sera say, "You are a wizard, right?" Ran tries to ask Sera if they have met before. Ran also feels the same when she sees the Unknown child.
Conan asks Haibara about Rum. She tells him that she has never met Rum in person, but has heard of his characteristics from other members. His description differs from person to person depending on who told the story.
Some said that he is a strong-built man. Some also said that he looks feminine. Others said that he is an old man. It is also possible that all of them were just body substitutes. Haibara also says that there is one characteristic about Rum that all the members who have met him can agree on, but she cannot remember it yet.
They then encounter a murder case where all three suspects happen to have characteristics like Rum's: a strong-built man, a feminine-looking man, and an old man.