During the eleventh Devil May Cry DMC Mission: The Order, you can collect one secret key: Cooper Key. DMC Copper Key can be found in a hidden room. This room is well hidden, and it is very easy to get lost in a set of tunnels that you enter upon defeating creatures in a hardware room.
Secret Key 1 – Copper KeyLocation: Mission 11 – The Order
Required ability/weapon: Eryx weapon
Additional Information: Upon defeating creatures in the room with hardware, you will end up in the crossroads. Choose the middle path for the Copper Key. Follow the path until you notice a lamp post that you can destroy. In a passage behind it, you will find the hidden room with a Red Crystal door blocking it. Proceed behind the lamp post, destroy red crystal with Eryx weapon, and acquire the Copper Key.