Sorrel, known as Professeur Chen in French-speaking regions, is a young Pokémon Professor aspirant from Veilstone City. He accompanies Ash Ketchum's journey to study Legendary Pokémon, partnered with his Lucario. His academic drive stems from childhood tragedy: a family Luxray sacrificed itself to protect him during a snowstorm, using its body heat to keep him warm overnight but perishing from the cold. This event caused Sorrel lasting difficulty forming bonds with Pokémon.

Guiding the group to Mount Tensei, Sorrel shares legends like Ho-Oh resurrecting three Pokémon into the Legendary beasts Entei, Raikou, and Suicune, and the prophecy of the "Rainbow Hero." He identifies the Rainbow Wing feather Ash carries as proof Ash is Ho-Oh's chosen challenger. His expertise includes interpreting the Rainbow Wing's glow near Ho-Oh and its fading color when held by someone possessing a "cold heart."

His character arc focuses on overcoming emotional barriers. After revealing his Luxray trauma during a campfire talk, he reaffirms his dedication to Pokémon research. This growth manifests when he aids Ash in the climactic battle against Marshadow and corrupted Pokémon, actively battling and strategizing. Following the Mount Tensei events, Sorrel departs to independently research Kanto's Legendary birds, specifically pursuing Articuno. He continues his partnership with Lucario, symbolizing his regained capacity to trust and collaborate with Pokémon.