Steven is a scientist who showed up in "Whichever Way the Wind Blows" to study Gloom's branched evolution line. He's best known for explaining to Ash and friends how Oddish can evolve into either Vileplume or Bellossom depending on certain conditions.
When the group first arrived, they broke up a fight between the two evolved forms, and Steven immediately thanked them for the intervention. He handed out masks to protect everyone from Stun Spore before diving into the real problem: a giant tree that served as the only shade in the area. Since both Vileplume and Bellossom needed it, they'd constantly chase each other back and forth—Bellossom would follow the shade when it covered the Vileplume's side, and vice versa, creating an endless cycle of conflict.
Steven warned the group about an incoming storm and theorized that the heavy winds might trigger more evolutions. Sure enough, the winds caused some Gloom to evolve, and afterward, he deliberately let the fighting resume to make a point: battling alone wouldn't solve anything. When Team Rocket showed up trying to capture the brawling Pokémon, they got hit with Stun Spore for their trouble. After Bayleef sent them blasting off and peace finally broke out among all the Vileplume and Bellossom, Steven waved goodbye to Ash, Misty, and Brock.
Content compiled by AnimeList.moe from publicly available sources.
