My theory:
Besides the FBI, CIA, MI6, and the Japanese Public Security Bureau, there may be another unknown organization—or at least a loose network of influential people—that has been secretly opposing Karasuma Renya and the Black Organization for decades.
Here are the reasons why I think this is possible.
1. Amanda Hughes
Amanda Hughes was not an FBI agent or a police officer, yet she knew an incredible amount about Rum and Karasuma.
She was an extremely influential political figure with international connections, showing that powerful civilians—not just intelligence agencies—were investigating the Black Organization.
2. Karasuma's Past
Amanda knew Rum from childhood because she had attended one of Karasuma's events.
This suggests that Karasuma once interacted with politicians, wealthy families, and other influential figures.
3. The Karasuma Mansion Incident
Many people died during the incident at Karasuma's mansion.
If surviving relatives, heirs, or business partners later discovered Karasuma's involvement, it would make sense that some of them dedicated themselves to opposing him in secret.
4. The Number of Powerful Families
Throughout the series, Gosho Aoyama repeatedly introduces wealthy and influential families (such as the Suzuki, Ooka, and Haneda families).
This may simply be world-building—but it could also be laying the groundwork for a larger hidden network.
5. The Mysterious Old Man
The mysterious old man who gives Rei Furuya orders has not been linked to any known organization.
If he is not simply another Public Security official, he could belong to a completely unknown group.
Arguments Against the Theory
No such organization has been officially mentioned.
The FBI, CIA, MI6, and PSB already serve as the main forces opposing the Black Organization.
These details may simply exist for world-building rather than foreshadowing a new faction.
Final Thoughts
I'm not claiming this theory is true.
However, when you combine Amanda Hughes, Karasuma's past, the Karasuma Mansion incident, the repeated focus on influential families, and the mysterious old man, I think there are enough hints to at least consider the possibility.
