There are many special achievements in "Wind Walker", and Pietro is one of them. If you want to achieve this achievement, you first need to find Pietro. He is located in the first biome of the whirlpool-Puppet Garden. You'll find him pretty quickly if not on your first run.
How to achieve Pietro the Wind Walker
Pietro
Rescuing Pietro on the adventure
Pietro is located in the Maelstrom's first biome, the Golem Garden.
You'll find him pretty quickly if not on your first run.
Once you find him, be sure to talk to him.
Pietro will let you give him a good whack on the head
As long as he does as he says, he will be taken back to the ark by the rescue fish.
He will then return to the Ark, where you can talk to him, where he will allow you to enter the vortex with the other Jumpers!