During the third Devil May Cry DMC Mission: Bloodline you can collect one secret key, the Copper Key. The Copper Key is in a hidden room near the Divinity Statue.
Secret Key – Copper KeyLocation: Mission 3 – Bloodline
Required ability/weapon: Arbiter (weapon, acquired in previous missions)
Additional Information: Near the Divinity Statue you can find a door with red marks on it. Smash them with Arbiter and proceed to the second floor of opened room. You will find the Copper Key on the second floor.