Thonky.com's Pokémon Legends: Z-A Walkthrough

After you reach rank V, you see a mysterious Pokémon that Emma believes is the 10% form of Zygarde.

Follow Zygarde

Keep following Zygarde. You can't catch it, and you also can't change your movement speed, so just run after it. Eventually you catch up to it.

Battle Mega Absol

Attack the Mega Absol to make Mega Power orbs fall out. Gather these orbs, then when you have enough of them, press the R control stick like a button to have Lucario Mega Evolve for you. Be sure to press Y to dodge the Mega Absol's attacks throughout the battle. Mega Absol sometimes sends a burst of air at you that you can dodge sideways with Y. Sometimes it creates a ring of red energy around itself, so run away from Mega Absol and press Y to move away from Mega Absol more quickly. Lucario's Mega Evolution might run out, so be sure to keep collecting orbs of Mega Power, and press the R control stick like a button again if Lucario is no longer Mega Evolved.

After the battle, you get Absolite, and the Absol joins you.

This starts Main Mission 10: The Secrets of Mega Evolution.