• English / United States: James' Mareanie
• Japanese / Japan: コジロウのヒドイデ
• Japanese (Romanized) / Japan: Kojirō no Hidoide
• Japanese (TL) / Japan: Kojiro's Hidoide

While searching underwater in scuba diving gear, James's Mareanie approached him and latched onto his helmet and its spikes went through the front glass viewing pane which almost impaled him and drowned him at the same time. He managed to make it back to dry land but Mareanie was still attached to the helmet. After a battle with Ash's Pikachu which they actually won, Mareanie continued to follow James and he decided to capture it.

It likes jumping on James's head and poisoning him because it makes him look like a Mareanie and more specifically he then resembles James's Mareanie first love.

One day, Mareanie while looking for a Corsola horn, it was attacked by a Tentacruel but then another Mareanie came to its rescue and tossed the Tentacruel far out into the sea. The Mareanie told James's Mareanie that it would get a Corsola horn for her. A few days later, it returned to the same spot to see that Mareanie who rescued it give a Corsola horn to another Mareanie. From that moment on, Mareanie with wracked with sorrow and had laid eyes on its mentor till they met again on the beach. It explained that it simply picked up a Corsola horn the other Mareanie dropped but James's Mareanie didn't believe it. It evolved into a Toxapex to show it cared and attacked James because it thought he was just toying with Mareanie's emotions. After James battled with the Toxapex, he vowed to do everything within his power to make Mareanie happy.

In the Top 16 round of the Alola League, James's Mareanie faced off against Jessie's Wobbuffet. The match was planned to be setup for Jessie to win but half way through the match, Mareanie decided that it wanted to battle for real and it fired up James and they were able to defeat Wobbuffet's defense to advance to the next round.