From Bulbapedia, the community-driven Pokémon encyclopedia.
Sullivan (Japanese: スズムラ Suzumura) is a character of the day who appeared in Whiscash and Ash. His English voice actor is Brian Maillard and his Japanese voice actor is 大川透 Toru Ohkawa.
A self-proclaimed "fishing god," Sullivan's ultimate goal is to capture a Whiscash named Nero. For years, Sullivan tried countless times to capture the Pokémon, but he always failed. He hoped that with the help of Ash and his friends, he would finally achieve his goal. Ash wanted to get Nero as well, since the Pokémon had eaten his Badge case.
Once Sullivan got Nero on land, he decided to use his secret weapon; a Master Ball. He threw it, and then the Water/Ground Pokémon opened its mouth and ate the Ball.
Although stunned that his Master Ball had been eaten, Sullivan vowed to never give up on trying to capture Nero, claiming the hunt would always be exciting for him.