Petasos was a Youma owned by Nephrite. Her essence was implanted into the hat of Kunitachi, the caretaker at a local park that was scheduled to be demolished to make room for a business complex. Petasos gave Kunitachi the power to control the animals at the park, allowing him to drive away the construction crews and protect nature, but every time he drew upon the Youma's power it caused his energy level to rise. When it finally peaked as he confronted Ami, Usagi, and Luna in the park, Petasos drained his energy and emerged from the hat.

Petasos fought by causing the animals in the park to attack, or by wrapping targets in vines that she fired from her hand or hair and tightening them until the victim was constricted to death. She trapped Sailor Mercury and Sailor Moon in vines, but they were rescued by Sailor Mars. Petasos then tried to attack the three Sailor Senshi again, but was stunned by a rose thrown by Tuxedo Mask, giving Sailor Moon the opportunity to destroy the Youma with Moon Tiara Action.