Sōji, also known as Sorrel, journeys alongside Ash in pursuit of the legendary Ho-Oh while striving to become a Pokémon Professor. His steadfast partner Pokémon, Lucario, accompanies him at all times.

During childhood, Sōji's parents were frequently absent due to work obligations, leaving the family Luxray as his primary guardian. On one occasion, he wandered into treacherous snowy terrain and sustained injuries. Luxray located him and shielded him with warmth throughout the night, sacrificing its own life to save him. This tragedy instilled in Sōji a profound fear of forming new bonds with Pokémon, driven by dread of repeating such loss. Lucario eventually became his first meaningful connection after this event.

Throughout the expedition, Sōji shares expertise on Pokémon legends, including the Rainbow Hero prophecy tied to Ho-Oh and Marshadow's role as Ho-Oh's agent. He discovers Ash’s abandoned Charmander and aids its recovery, later clarifying the Rainbow Wing’s purpose as a guide bestowed only on those Ho-Oh deems worthy. When confronting aggressive Primeape, Sōji cautions against retaliation to prevent escalating hostilities. After Ash’s Metapod evolves into Butterfree, he identifies behavioral signs indicating a pink Butterfree is female and explains Ash might need to release his Butterfree for it to follow natural instincts.

Sōji later discloses Bonji’s research on Ho-Oh and Marshadow’s function as a "shadow guide." At Mount Tensei, he warns that placing the corrupted Rainbow Wing by someone harboring an "evil heart" will invite disaster. After Cross triggers this corruption, Sōji assists in defending Ash against Marshadow’s brainwashed Pokémon army. Post-conflict, he resolves to study the legends of Kanto’s Legendary Birds—Articuno, Zapdos, and Moltres. The journey culminates with Sōji and Lucario witnessing Articuno, cementing his dedication to Pokémon research.