Pierre is a character of the day who appeared in Type Casting.

He and Marie were trying to determine whether Sudowoodo is a Rock-type or a Grass-type. However, their co-written research paper became sidetracked by their conflicting beliefs. Pierre thought that Sudowoodo was a Rock-type Pokémon because it is written as such in his reference book.

With Ash and his friends' help, the two researchers were able to find a Sudowoodo and test their theories in their laboratory. He impulsively sprayed water from a hose at Sudowoodo to prove his point, causing it to flee. After a battle against Team Rocket, Marie's hypothesis was proven false when she realized that Sudowoodo had used Mimic to copy Pokémon attacks, including Grass-type moves. While Pierre eventually proved he was right, he and Marie apologized to each other and vowed to continue working together.

(Source: Bulbapedia)