The following page of our guide to Devil May Cry 5 contains a description of the ninth secret mission in the game.
Secret Mission 09
Where to find it: Mission 12 - Yamato
When you destroy all the blood clots, go back to the fountain and jump off to the underground. When you take the stone steps to the top, turn around and you will see a platform - there, you will find the Secret Mission 09.
Here, Dante must spend 15 seconds in the air without touching the ground. This mission should be performed in the mode Trickster mode, bouncing up off the wall and spamming the cavaliere attack in the air, all the time. Attacks by motorcycle keep Dante in the air. If you start falling to the ground, try to hit an enemy blow to keep in the air. As a reward, you'll get a blue fragment of the sphere.