During the second Devil May Cry DMC Mission: Home Truths, you can collect two secret keys: Copper Key and Argent key. The Copper key is located inside the house, in the first huge room.
You don’t need any additional skills for collecting this key.
Secret Key 2 – Copper KeyLocation: Mission 2 – Home Truths
Additional Information: Immediately after the mission starts, walk forward to the huge room with stairs, go up the stairs, then turn right. The Copper Key is located at the end of the hall.