Nero first gained the attention of a fisherman named Sullivan. Impressed by its gargantuan size, he was determined to capture it. However, after fifty years of trying to catch the elusive Whiscash, Sullivan's efforts remained fruitless.
In Whiscash and Ash, Ash wanted to catch Nero as well, but for a very different reason. While resting at the lake in which Nero resided, the Pokémon swallowed Ash's Badge case and refused to give it back. The young Trainer was desperate to get back his case, as well as the five Badges he had worked so hard to earn.
While watching Ash and his friends fish with Sullivan, Team Rocket decided to steal Nero as a present for Giovanni. However, their plans were foiled with relative ease. With the help of Pikachu's Thunder attack on Team Rocket, Nero flew out of the water and spat out Ash's Badge case. Ash was delighted and relieved to be reunited with his precious Badges. Sullivan, however, was not as lucky as Ash was. While attempting to catch Nero with a Master Ball, the Pokémon ate the Ball and jumped back into the lake.
Sullivan promised that he would keep trying to capture Nero and Ash and his friends bid farewell to the fisherman. Nero was last seen playing with the Master Ball on its tongue on the lake's surface.
Nero is at least 100 years old and, according to Sullivan, has ruled the lake for that amount of time. While an ordinary Whiscash is just under three feet tall, Nero is significantly larger than this. Nero, like all Whiscash, has an affinity for objects that shine and sparkle, such as Ash's Badge case, which is why Nero stole it in the first place. Since Nero kept showing the Badge case to Ash and then retracting its tongue in a taunting manner, it can be concluded that Nero enjoys making others angry for its own amusement. May pointed out that Nero seemed to be laughing at Ash on more than one occasion in response to the latter yelling at it in frustration.