Neesha, along with Corey, Fergus, and Ash and his friends were the only Pokémon Trainers who were able to make it to New Island. She made it to New Island by riding on her Dewgong's back. She was one of the Trainers who battled Mewtwo (or rather, its Blastoise clone), using Shellshocker, her Blastoise; however, Mewtwo's cloned Blastoise easily defeated it with a Rapid Spin attack.

Like those of the other Trainers on New Island, all of Neesha's Pokémon were cloned by Mewtwo.

(Source: Bulbapedia)