From Bulbapedia, the community-driven Pokémon encyclopedia.
Benny (Japanese: テル Teru) is a character of the day who appeared in Tricks of the Trade, and the original Trainer of Wobbuffet.
He traveled to Palmpona for its annual Pokémon Swap Meet, hoping to trade his Wobbuffet for a new Pokémon to raise. Ash and his friends tried to help Benny locate a Trainer willing to trade with him; he briefly contemplated asking if Ash would be interested in trading his Tauros, but decided not to after he overheard Ash saying that he wasn't there to trade. A Trainer with a Stantler initially wanted to trade with Benny, but bailed at the last minute when he decided he would rather have a Flying-type Pokémon.
Meanwhile, the Magikarp salesman—running away from James—bumped into Jessie and sent her flying. She bumped into a nearby trading machine with Benny's Wobbuffet already in it, and Jessie's Poké Ball containing her Lickitung fell into the machine. Unbeknownst to her, the trade went through, and Jessie, not realizing what happened, took the Poké Ball containing Wobbuffet and ran off.
By the end of the episode, Benny was showing off his new Lickitung to Ash and his friends. He called into the distance, promising to Jessie that he would take care of Lickitung and hoping that she would take care of Wobbuffet in return.