In "The Legend of Thunder," Jimmy, Marina, and Vincent are assisted by a character named Eusine in their battle against Attila and Hun. Despite wearing a cape, which catches the eye of Marina, Eusine is close friends with Morty, who acts as his voice of reason. However, when Eusine suddenly disappears at the end of the episode, Mortys lack of surprise suggests that this has happened before.
Eusine was first mentioned by Morty, who revealed that he had asked him to use his powers to find Suicune, but with no success. After Morty's defeat by Suicune, Eusine set out on his own to catch it. During his hunt, he encountered Crystal and accidentally shocked Arckee with Electrode, declaring them rivals before disappearing.
Later, Eusine tricked an old couple into giving him Moomoo Milk by "curing" their Miltank. However, Crystal followed him and discovered that he had put a berry in its mouth and manipulated the kind couple. Despite this revelation, Eusine seemed unconcerned.
Finally, Eusine and his companions encountered Suicune, only to discover that it was actually a Ditto sent by Team Rocket to distract them. Nonetheless, they were able to let Suicune escape, and Eusine defeated the Team Rocket Elite Trio.