Warning! James' Mareanie might evolve in one of the future episodes or chapters. Please do not insert any speculation, nor from the confirmed synopsis until the episode or chapter in which the Pokémon evolves has aired or been released.
Mareanie first appeared attacking James because she wanted to eat his Corsola-shaped helmet, but later felt disappointed when she found out that she was only a helmet. After Jessie, who was attacked by her tentacles, gave James a push, she poisoned him, causing him to look like another kind of her species. Thus she blushed and started to chase him, much to his dismay. However, she later helped Team Rocket and she was so strong that she managed to poison Ash's Rowlet.