Magearna originally resided at an Alola antique shop, still in its inactive state. It later was bought by Mohn, who believed it was made by scientists to protect a princess of a far-away land; as he thought of his little princess, baby Lillie, but was unable to activate it and he took notes of it, and drew pictures of how it could play with Lillie.
In the present day after Mohn went missing, his wife Lusamine showed Magearna to Lillie and Gladion when she brought them to his old room where it resides. When Lillie saw it and his old Z-Ring, Magearna listened when she told its desire to see her father again then it briefly lit up and she decided to activate it herself until he was found.
Magearna was seen still inactive in Lillie's closet. While Lillie was getting herself ready for school, she told Magearna that she'll see it later. Before it recently awakened, Lillie introduced Magearna to her friends.
Later, Lillie put Magearna's Soul-Heart in its chest, before falling asleep. That next day, Magearna was still inactive, but then she noticed a page with it and her father's Zoroark and asked Gladion to have their father's partner come out. After being shown of one of Zoroark's memories of Lillie taking her first steps as a baby because she wanted to feel it, Magearna finally reactivated. Magearna gave Lillie flowers before receiving a hug from her, as she was very happy that she finally got to meet it.
After being introduced to everyone, Magearna accompanied Lillie and her family in their quest to finding Mohn.
None of Magearna's moves are known.
- Magearna is one of the three Mythical Pokémon to be with a main character; the first being Shaymin, who Mallow watched over and Ash's Melmetal, a fully owned Mythical Pokémon.